Glossary of Eye and Eye Health Related Terms & Definitions
In an effort to help educate our patients about a variety of eye and eye health related topics, we offer a list of vision related terms. Please note this list is for general, educational information only and not for the purpose of providing on-line medical advice, diagnosis or treatment of any eye health condition or disease. If you have any concerns about your vision or eye health, please contact a licensed eye care professional immediately for assistance.
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | R | S | T | U | V | W | X | Z
A
aberrometer A device that can identify common and more obscure vision errors by measuring the way light waves travel through the eye's optical system.
ablation Removal. In vision, ablation refers to the surgical removal of eye tissue to correct a refractive error such as myopia. For example, in laser procedures such as LASIK and PRK, the excimer laser ablates, or removes, tissue from the cornea.
accommodation Eye's ability to automatically change focus from seeing at one distance to seeing at another.
accommodation disorder Accommodation refers to the eye's ability to automatically change focus from seeing at a distance to seeing at near. Accommodation disorders have a variety of causes. Symptoms include blurred vision, double vision, eyestrain, headache, fatigue and difficulty concentrating (particularly while reading). Presbyopia is an accommodation disorder that affects everyone if they become old enough, since its causes relate to the aging of the eyes.
acetate Type of plastic often used in eyeglass frames.
acrodermatitis enteropathica Rare inherited condition characterized by dermatitis, hair loss, diarrhea and zinc deficiency. The most common eye symptom is light sensitivity, but conjunctivitis and other symptoms may also be present.
AK (astigmatic keratotomy) Procedure in which a surgeon cuts the cornea so that it is more spherical when it heals, thus reducing astigmatism.
albinism Condition where a person or animal lacks pigment. Albinos' eyes often have very light blue or pink irises and a pink pupil (due to lack of pigment inside the back of the eye). Visual symptoms include light sensitivity, nystagmus, blurred vision, vision loss and strabismus.
allergy Reaction of the body's immune system to a foreign substance (e.g., pollen, animal dander, etc.). When the eyes are affected, the most common symptoms are redness, itching, chemosis, tearing, swollen eyelids and stickiness.
amaurosis fugax Sudden and usually temporary vision loss caused by an "eye stroke." When a clot or blockage disrupts blood flow to the eye, symptoms can include curtain-like darkness, usually in just one eye.
amblyopia Also called lazy eye. Undeveloped central vision in one eye that leads to the use of the other eye as the dominant eye. Strabismus is the leading cause, followed by anisometropia.
AMD or ARMD (Age-Related Macular Degeneration) Disorder characterized by the gradual loss of central vision due to a damaged macula (which is made up of retinal cones necessary for sight).
angiogenesis The formation of new blood vessels in the body. Also see neovascularization.
aniridia Absent or partially absent iris, typically congenital. Additional symptoms include poor vision and photophobia.
ANSI The American National Standards Institute is a private, non-profit organization that coordinates efforts to develop standards for manufacturing many different products, including eyeglass lenses. For example, certain ANSI standards define acceptable levels of impact resistance for safety eyewear.
ANSI Z87.1-2003 Standard The American National Standards Institute's Practice for Occupational and Educational Eye and Face Protection; eyewear that meets this standard is considered safer than eyewear that does not.
anterior chamber Part of the eye behind the cornea and in front of the iris and lens.
antibody A protein activated by the body's immune system that fights infection, toxins and other foreign substances.
antioxidant Substance that inhibits oxidation and can guard the body from damaging effects of free radicals. Molecules with one or more unpaired electrons, free radicals can destroy cells and play a role in many diseases. Antioxidant vitamins include B, C and beta carotene. Antioxidants may help prevent macular degeneration and other eye diseases; many studies are in progress.
anti-reflective coating (AR coating) Thin layer(s) applied to a lens to reduce the amount of reflected light and glare that reaches the eye.
aqueous humor Clear fluid in the front of the eye, between the cornea and the iris, that provides nutrients to the cornea and the lens. The fluid is produced by the ciliary body. Glaucoma causes a difficulty in draining this fluid, and pressure builds up. The result is damage to the optic nerve and loss of vision.
aspheric Not quite spherical. Aspheric eyeglass lenses are popular among people who have strong prescriptions because they are thin and lightweight, and reduce distortion and eye magnification. Aspheric contact lenses can work as a multifocal, or to correct a single-vision problem like astigmatism.
astigmatism Condition in which the cornea's curvature is asymmetrical (the eye is shaped like a football or egg instead of a baseball); light rays are focused at two points on the retina rather than one, resulting in blurred vision. Additional symptoms include distorted vision, eyestrain, shadows on letters, squinting and double vision.
aviator glasses These frames have a large upside-down teardrop shape and usually have a double-bar bridge. Originally, these were metal-framed sunglasses worn mainly by pilots, but now the shape also comes in plastic and is used for eyeglasses as well.
B
best corrected visual acuity (BCVA) The best vision you can achieve with correction (such as glasses), as measured on the standard Snellen eye chart. For example, if your uncorrected eyesight is 20/200, but you can see 20/20 with glasses, your BCVA is 20/20.
beta blocker Drug that widens or dilates blood vessels, thus enabling more normal flow of blood. Topical beta blockers applied as eye drops also can lessen fluid production and lower internal eye pressure (intraocular pressure) in eye diseases such as glaucoma to reduce the possibility of optic nerve damage. Beta blockers also are used to control high blood pressure (hypertension). Side effects can include respiratory problems.
beta titanium Titanium alloy. Beta-titanium eyeglass frames are both strong and flexible.
bifocal Lens with one segment for near vision and one segment for far vision. The term "bifocal" can apply to both eyeglass lenses and contact lenses.
binocular vision Ability of both eyes to work together to achieve proper focus, depth perception and range of vision.
biocompatible Able to coexist with living tissues without harming them.
bioptic telescopic lenses (BTL) Devices attached to glasses that provide extreme magnification, typically used for driving. For people with low vision who are qualified, telescopic lenses are attached above the driver's line of sight to help magnify objects such as road signs.
blepharitis Inflammation of the eyelid(s), typically around the eyelashes. Various types of dermatitis, rosacea and allergic reactions can cause blepharitis. Symptoms include a red or pink eyelid, crusty lid or lashes, burning, foreign body sensation, eye or eyelid pain or discomfort, dry eyelid, dry eye, eyelash loss, grittiness, stickiness, eyelid swelling and tearing.
blepharoconjunctivitis Inflammation of the eyelid and conjunctiva. Infections and allergic reactions are among the causes. Symptoms include a red or pink eye, a red or pink eyelid, pain or discomfort of the eye or around the eye, tearing, burning, eye dryness and eye stickiness.
Bowman's membrane Corneal layer between the epithelium and the stroma.
bridge The part of eyeglasses that extends across the nose.
C
cable temple Style of eyeglasses that wraps around the ear, to keep them well-fastened.
capsulotomy In cataract surgery, an incision into the capsule that contains the natural crystalline lens of the eye, in order to remove and replace the natural lens with an artificial one.
cataract Clouding of the natural lens of the eye, usually caused by aging in conjunction with other risk factors, such as exposure to the sun's UV rays, smoking, steroid intake and diabetes. Symptoms include blurred vision, glare, halos around lights, colors that are less bright, a cloudy spot in your vision and, sometimes, temporary vision improvement.
chalazion A small bump on the eyelid caused by an obstructed meibomian gland. Additional symptoms include light sensitivity, tearing and eyelid swelling. Chalazia are usually not painful unless they become infected.
choroidal neovascularization Abnormal growth of new blood vessels in the choroid. Choroidal neovascularization is commonly associated with macular degeneration, but it can occur as a result of other eye conditions as well. Symptoms include vision loss and metamorphopsia.
ciliary body Part of the eye between the iris and the choroid; the three form the uvea. The ciliary body's main functions are accommodation, aqueous humor production and holding the lens in place.
CK (Conductive Keratoplasty) Procedure in which a surgeon uses radio waves to heat collagen in the cornea's periphery to shrink it and reduce hyperopia (farsightedness). CK is also used to treat presbyopia.
clip-on Type of glasses that attaches to your regular glasses, such as clip-on sunglasses.
color blindness Partial or total inability to distinguish specific colors. Color blindness is inherited, and is much more common in men than in women.
computer vision syndrome Collection of problems, mostly eye- and vision-related, associated with computer use. Symptoms include eyestrain, dry eyes, blurred vision, red or pink eyes, burning, light sensitivity, headaches and pain in the shoulders, neck and back.
cone A photosensitive receptor in the retina that helps you to see color.
conjunctiva The clear, thin mucous membrane that lines the "white" of the eye (sclera) and the inner surface of the eyelid.
conjunctivitis Inflammation of the conjunctiva, characterized by a pink eye. The cause is either infectious or allergic, though the term "pink eye" is commonly used for any type of conjunctivitis. Other symptoms include burning, discharge, dryness, itching, light sensitivity, eye pain or discomfort, stickiness, tearing and chemosis.
contact lens drops Eyedrops for contact lens wearers; regular eyedrops can discolor contact lenses.
contact lens problem Contact lens problems can range from minor to sight-threatening, and include protein build-up, debris on the lens, a ripped or nicked lens, infections and more. Symptoms can include frequent blinking, blurred vision, burning, discharge, foreign body sensation, itching, light sensitivity, eye pain or discomfort, a red or pink eye or lid and eyelid swelling.
contrast sensitivity The ability of the eye to detect the line of demarcation between an object and its background or an adjacent object.
convergence Eyes' ability to turn inward. People with convergence insufficiency have trouble (eyestrain, blurred vision, etc.) with near tasks such as reading.
cornea The clear part of the eye covering the iris and pupil; it lets light into the eye, permitting sight.
corneal abrasion A loss of the epithelial layer of the cornea, typically due to minor trauma (contact lens trauma, a sports injury, dirt or another foreign body, etc.). Symptoms include blurred vision, foreign body sensation, grittiness, light sensitivity, eye pain or discomfort, a red or pink eye and tearing.
corneal dystrophy One of a group of conditions, usually hereditary, in which the cornea loses its transparency. The corneal surface is no longer smooth. Common forms include map-dot-fingerprint dystrophy, Fuch's dystrophy and lattice dystrophy. Symptoms include blurred vision, foreign body sensation, light sensitivity, eye pain or discomfort and vision loss.
corneal edema Swelling of the eye's cornea; causes include intraocular surgery, corneal dystrophies, high intraocular pressure and contact lens complications. Symptoms include vision loss, halos around lights, a white or cloudy spot on the eye, photophobia, eye pain and foreign body sensation.
corneal erosion Recurrent breakdown of the corneal epithelium, typically caused by a previous corneal abrasion or by map-dot-fingerprint dystrophy. Symptoms include blurred vision, foreign body sensation and eye pain or discomfort.
corneal implants Devices (such as rings or contacts) placed in the eye, usually to correct vision.
corneal opacity A cloudy spot in the cornea, which is normally transparent. Causes include corneal scar tissue and infection. Symptoms include halos around lights, photophobia, vision loss and a white or cloudy spot on the eye.
corneal ring Type of vision correction surgery where a doctor inserts a tiny plastic ring into the cornea (which lets light into the eye). This ring reshapes the cornea, helping it to focus light better onto the retina so you can see better. The ring can be adjusted and even removed if desired.
corneal topography A corneal topographer shines light onto the surface of the eye, then measures the reflected light to create a map of the cornea's curvature as well as any irregularities. The map is used for evaluations related to refractive surgery, contact lens fitting and corneal disease management. It is especially useful for measuring astigmatism. The color map uses blue and green to represent flatter areas of the cornea, while red and orange represent steeper areas.
corneal ulcer An infected corneal abrasion. Frequently found in extended wear contact lens wearers. A corneal ulcer is an ocular emergency. Symptoms include light sensitivity, eye pain or discomfort, a red or pink eye, a white or cloudy spot on the eye and tearing.
crossed eyes Type of strabismus (a misalignment of the eyes) where one or both eyes point inward, toward the nose.
D
dacryostenosis Blocked tear duct, which is characterized by a lot of tearing; you may also have a discharge or a sticky eye.
daily wear These soft contact lenses are worn every day for six months up to a couple of years. They require daily cleaning and disinfecting, as well as a periodic enzymatic soak (usually once a week).
decentered ablation Also called decentration. Refractive surgery complication in which the laser is not centered on the pupil when it removes tissue. Symptoms include glare, double vision and halos.
Descemet's membrane Corneal layer between the stroma and the endothelium.
diabetic maculopathy Eye disease related to diabetes that creates swelling and abnormal leakage of fluids and fats into the macula, where fine focusing and central vision occur. Because of the location of eye damage, severe diabetic maculopathy can cause loss of central vision and blindness. Another form of diabetic eye disease is diabetic retinopathy.
diabetic retinopathy Leaking of retinal blood vessels in advanced or long-term diabetes, affecting the macula or retina. Most people have no symptoms at first, but can develop blurred near vision, double vision, floaters, retinal/vitreous hemorrhages and metamorphopsia. In later stages, you can also suffer vision loss.
diabetic vitreous hemorrhage Bleeding into the gel-like (vitreous humor) interior of the eye, originating from blood vessels weakened by diabetes. Because blood in the vitreous can block vision, a vitrectomy may be required.
diopter Unit of measure for the refractive (light-bending) power of a lens; eye care practitioners use it in eyeglass and contact lens prescriptions. A negative number refers to nearsightedness; a positive number, farsightedness. For example, someone with -8.00 diopter lenses is very nearsighted, while someone with +0.75 diopter lenses is only slightly farsighted.
disposable contact lenses Technically, this is any contact lens that is thrown away after a short period of time. Among most eye care practitioners, "disposable" usage ranges from one day to two weeks, while "frequent replacement" lenses are discarded monthly or quarterly.
Dk/t Dk is the oxygen permeability of a contact lens material; t is the thickness of the contact lens design. Dk/t is a measurement of a contact lens's oxygen transmissibility.
double vision Also called diplopia. When two images of the same object are perceived by one or both eyes.
drainage angle In glaucoma terminology, "angle" refers to the drainage channel for the aqueous humor in the eye; improper drainage can lead to the high intraocular pressure associated with glaucoma. In narrow-angle glaucoma, the channel is blocked, whereas open-angle glaucoma has other causes, such as the body producing too much aqueous humor.
drooping eyelids Also called ptosis. Condition in which the upper eyelid(s) only sag. It can be present at birth or caused by a later problem with the muscles lifting the eyelid, called levators.
druse Small yellow or white deposit in the eye. Drusen are sometimes signs of macular degeneration.
dry eye Lack of sufficient lubrication and moisture in the eye. Most dry eye complaints are temporary and easily relieved; dry eye syndrome, also called keratoconjunctivitis sicca, is chronic and needs more advanced treatment by an eye care practitioner.
dry eye syndrome Chronic dryness due to reduced quality or quantity of the eye's tear film, or due to increased evaporation of the existing tear film. Dry eye syndrome has many causes, including aging, certain systemic diseases and long-term contact lens wear. Additional symptoms include foreign body sensation, eye pain or discomfort, burning, grittiness, itching, light sensitivity, frequent blinking, a red or pink eye and tearing.
dry skin Skin that is generally dry may include dry eyelids.
E
edema Accumulation of an excessive amount of watery fluid, which causes swelling.
endophthalmitis Inflammation of the interior of the eye, typically caused by an infection from eye surgery or trauma. Endophthalmitis is an ocular emergency. Symptoms include floaters, light sensitivity, eye pain or discomfort, a red or pink eye and vision loss.
endothelium The cornea's inner layer of cells.
endpiece At left and right of the front of an eyeglass frame, the part that attaches to the temples, usually but not always with a screw mounting.
enzymatic cleaner A cleaner that removes protein deposits and other debris from contact lenses. It's recommended for use either daily, weekly, or monthly. Some enzymatic cleaners are a small tablet dropped into a solution along with the lens; others come in liquid form.
epiretinal membrane Thin layer of scar tissue on the retina; also called a macular pucker. Epiretinal membranes have a variety of causes, including vitreous detachment, but the cause is often unknown. In its early stages, an epiretinal membrane is often asymptomatic, but some people have blurred vision. You may also develop metamorphopsia.
episclera Outer layer of the eye's sclera that loosely connects it to the conjunctiva.
episcleritis Inflammation of the episclera. The cause is usually unknown, but episcleritis may be associated with some systemic (e.g., autoimmune) diseases. Symptoms include a red or pink eye, eye pain or discomfort, light sensitivity and tearing.
epithelial ingrowth LASIK complication in which epithelial cells grow under the LASIK flap; epithelial ingrowth does not usually affect vision.
epithelium The cornea's outer layer of cells.
esotropia When one or both eyes point inward, so the eyes are "crossed." This is one type of strabismus.
excimer laser An instrument that uses shorter wave (ultraviolet) light to vaporize and remove tissue from the eye's surface during vision correction procedures.
exotropia When one or both eyes point outward; also called "walleyed." This is one type of strabismus.
extended wear Currently, these contact lenses are FDA-approved to be worn without removal for up to seven days (or 30 days in the case of one brand), meaning some people will be comfortable sleeping with them in their eyes. Thirty-day contact lenses are sometimes referred to as "continuous wear."
eye tumor A growth or mass that occurs in or next to the eye. Specific tumors, both benign and malignant, include the dermoid cyst, capillary hemangioma, cavernous hemangioma, choroidal melanoma, retinoblastoma, rhabdomyosarcoma and lymphoma. The cause is dependent on the type of tumor you have. Symptoms can include blurred vision; a bulging eye; double vision; floaters; foreign body sensation; pain or discomfort in the eye, the lid or around the eye; swelling of the lid or around the eye; a red or pink eye; ptosis; vision loss; limited eye or lid movement; a white or cloudy spot on the eye; and an iris defect.
F
farsightedness Also called hyperopia. To farsighted people, near objects are blurry, but far objects are in focus.
femtosecond laser Device that creates bursts of laser energy at an extremely fast rate measured in terms of a unit known as a femtosecond (one quadrillionth of a second). These ultra fast energy pulses precisely target and break apart tissue or other substances at a molecular level, without damaging adjacent areas.
fixation In terms of vision, the eye's ability to maintain gaze upon an object.
floaters A dark or gray spot or speck that passes across your field of vision and moves as you move your eye. Floaters are very common and may look like clouds, strands, webs, spots, squiggles, wavy lines or other shapes. As your eye ages, the gelatinous vitreous humor begins to liquefy in the center of the gel. Floaters are caused by the undissolved vitreous humor that floats in the liquid vitreous. Sometimes, a "shower of floaters" is a sign of a serious condition, particularly if you also see flashes of light.
fluorescein Compound that becomes a bright, fluorescent yellow-green when in contact with alkaline substances. A fluorescein dye solution can help eye doctors see corneal lesions or conduct tests for eye dryness.
forceps Surgical tool shaped like tongs, used for gripping.
foreign body Something in or on the eye that doesn't belong there. Symptoms include foreign body sensation, eye pain or discomfort, a red or pink eye, tearing, frequent blinking, blurred vision, discharge, light sensitivity and vision loss.
foreign body sensation Sensation that something is in your eye.
fovea A depression in the retina that contains only cones (not rods), and that provides acute eyesight.
fungal keratitis eye infection The source of a 2006 outbreak of fungal eye infections among contact lens wearers is a fungus known as Fusarium, found in places such as soil, water, and organic matter including plants. Ordinarily, it is rare for this fungus to invade and damage the eye. But symptoms can be severe, and if untreated, the infection may become so eye-damaging that a corneal transplant is required.
G
glaucoma Disease characterized by elevated intraocular pressure, which causes optic nerve damage and subsequent peripheral vision loss. Most people have no initial symptoms of chronic (open-angle) glaucoma, but you can develop peripheral vision loss, headaches, blurred vision, difficulty adapting to darkness and halos around lights. Other forms of glaucoma (e.g., closed-angle glaucoma) may have additional symptoms such as eye pain, a pupil that doesn't respond to light, redness, nausea and a bulging eye.
Graves' ophthalmology Autoimmune eye disorder usually associated with abnormalities of the thyroid gland; symptoms include eyelid retraction, bulging eyes, light sensitivity, eye discomfort, double vision, vision loss, a red or pink eye and a limited ability to move the eyes.
H
hard contact lenses Rarely worn now, these are the small, hard lenses made of PMMA material that many people wore in the '70s and '80s. Compared with modern soft and rigid lenses, they are less healthy to wear long-term, since the material doesn't allow oxygen to reach the surface of the eye.
higher-order aberration Irregularity of the eye other than a refractive error (myopia, hyperopia or astigmatism). Higher-order aberrations sometimes affect your vision (such as decreasing contrast sensitivity), and sometimes do not.
high-index Type of lens with a higher index of refraction, meaning that light travels faster through the lens to reach the eye than with traditional glass or plastic. It is denser, so the same amount of visual correction occurs with less material (whether glass or plastic) - so the lens can be thinner.
histamine A protein that can be released as part of the body's immune system responses during an allergic reaction. Presence of histamine can lead to inflammation and swelling, which is why antihistamines often are prescribed for allergy symptoms.
histiocytosis Abnormal proliferation of histiocytes (immune system cells). Common symptoms include bone tumors and skin rashes. If histiocytosis affects the eyes, it causes bulging.
homocysteine An amino acid containing sulfur. A high homocysteine level in the blood is a possible risk factor for heart disease. One major study indicates that lowering homocysteine levels through vitamin B supplementation might help prevent diseases associated with impaired function of small blood vessels, such as macular degeneration. More studies are needed to verify such a link.
hyperopia Also called farsightedness. Condition in which the length of the eye is too short, causing light rays to focus behind the retina rather than on it, resulting in blurred near vision. Additional symptoms include eyestrain and squinting.
hypoglycemia Abnormally low level of sugar (glucose) in the blood, which causes dizziness, hunger, shakiness and other symptoms. Skipping or delaying meals, too-small meals, or a high level of physical activity can cause hypoglycemia. Too much alcohol or certain drugs can also cause it. In diabetics, it can be caused by too much insulin.
hypotony Low intraocular pressure, often caused by eye surgery or trauma. Symptoms include blurred vision and eye pain or discomfort.
I
intraocular lens (IOL) Artificial lens that a cataract surgeon places in a patient's eye after removing the eye's natural lens. Like a contact lens, it has a built-in refractive power tailored specifically to the patient's visual condition.
intraocular pressure (IOP) Eye pressure, as determined by the amount of aqueous humor filling it. High IOP (ocular hypertension) can be a sign of glaucoma.
iris A pigmented membrane that lies between the cornea and the lens; it acts as a diaphragm to widen or narrow the opening called the pupil, thereby controlling the amount of light that enters the eye.
iritis Inflammation of the iris.
ischemia Poor blood flow. Obstructions such as clots in veins and arteries can block blood flow, depriving tissue of oxygen and nutrients. These blockages also can cause "eye strokes" and sudden vision loss.
J
jaundice Yellow coloring in the skin and eyes caused by high levels of a pigment called bilirubin. Jaundice is associated with a variety of conditions involving the liver, gallbladder and bile ducts, including hepatitis and cirrhosis.
K
keratectomy Surgical removal of part of the cornea.
keratitis Inflammation of the cornea, caused by an infection or inflammatory process. Symptoms include eye pain or discomfort, light sensitivity, foreign body sensation, grittiness and tearing.
keratoconjunctivitis Inflammation of the cornea and conjunctiva.
keratoconjunctivitis sicca Also called dry eye syndrome. Chronic lack of sufficient lubrication and moisture in the eye.
keratoconus Degeneration and thinning of the cornea resulting in a cone-shaped bulge (a type of irregular astigmatism). The cause is unknown, but may be genetic. The first symptom is blurred vision that doesn't improve enough with glasses (contacts usually work well for a while). You may also have double vision or distorted vision.
keratoplasty Any of several types of corneal surgery, such as shrinking the collagen to reduce farsightedness or transplanting a new cornea to treat keratoconus.
keratotomy Incision of the cornea.
keratometer An instrument that measures the curvature of the eye's clear, front surface (cornea). Keratometers help eye doctors collect information for contact lens fittings and surgical procedures. With keratometry, reflected images also can help identify dry eyes.
L
lacrimal plug Also called punctal plug. Device to block the lacrimal punctum (an opening at the end of a tear duct), to keep the eye moist.
laser photocoagulation Procedure in which a surgeon uses a laser to coagulate tissue, usually to seal leaking blood vessels and destroy new ones in diseases like macular degeneration and diabetic retinopathy.
LASEK (Laser Epithelial Keratomileusis) Procedure that is similar to LASIK, except that the surgeon cuts a flap in the epithelium only, instead of through the epithelium and part of the stroma. LASEK is used mostly for people with thin or flat corneas who are poor candidates for LASIK, which requires more corneal tissue for success.
LASIK (Laser-Assisted In Situ Keratomileusis) Surgical procedure in which a tiny flap is cut in the top of the cornea, underlying corneal tissue is removed with an excimer laser, and the flap is put back in place. LASIK corrects myopia, hyperopia, astigmatism, and presbyopia through monovision.
lateral rectus muscle Muscle that moves the eye away from the nose.
lens 1. The nearly spherical body in the eye, located behind the cornea, that focuses light rays onto the retina. 2. A device used to focus light into the eye in order to magnify or minify images, or otherwise correct visual problems. Eyeglass lenses, contact lenses, and intraocular lenses are examples. Sometimes a lens provides only a cosmetic benefit, as in non-correcting color contacts, theatrical contact lenses, or contacts that hide a disfigurement of the eye. Other times a lens protects the eye, as in safety glasses or sunglasses. A bandage contact lens may be applied after eye surgery. And contact lenses used in ortho-k or corneal refractive therapy are designed to reshape the cornea for better vision when the lens is removed.
lens dislocation Full or partial displacement of the eye's lens. Dislocation is often caused by trauma to the eye or head, but may also be inherited or come as the result of certain systemic conditions, such as Marfan's syndrome or homocystinuria. Blurred vision is a typical symptom, and some people may experience double vision and/or develop glaucoma.
leukocoria White pupil. Causes include congenital cataract, retinoblastoma, intraocular infection, Coat's disease and retinopathy of prematurity.
limbus Boundary area connecting the cornea and sclera; the three form the eye's outermost layer.
low vision Also called partial sight. Sight that cannot be satisfactorily corrected with glasses, contacts, or surgery. Low vision usually results from an eye disease such as glaucoma or macular degeneration.
LTK (Laser Thermal Keratoplasty) Also called Laser Thermokeratoplasty. Surgery to correct mild farsightedness in people over 40; the doctor uses a holmium laser to heat the cornea and shrink its collagen.
M
macula Part of the eye near the middle of the retina; the macula allows us to see objects with great detail.
macular degeneration Disorder characterized by changes in the eye's macula that result in the gradual loss of central vision. The exact cause is unknown, but appears to be related to a genetic predisposition, smoking and several other risk factors. Central vision may be blurred, distorted (metamorphopsia) or shadowy before vision loss occurs.
macular edema Swelling of the central portion of the retina (macula), due to buildup of fluid leaking from retinal blood vessels. Causes temporary or permanent vision loss if untreated.
macular hole Hole in the eye's macula; many doctors believe it can be caused by vitreous shrinkage as we age. Symptoms include blurring or a blind spot in central vision and metamorphopsia.
maculopathy Disease of the macula, such as age-related macular degeneration.
madarosis Eyelash or eyebrow loss. Causes include infections, metabolic disorders, blepharitis, certain drugs, lupus erythematosus and trauma.
medial rectus muscle Muscle that moves the eye toward the nose.
meibomian gland Gland found in the eyelid that produces the oily outer layer of the three-layer tear film that lubricates the eye.
meibomianitis Inflammation of the meibomian glands; rosacea is a common cause. Symptoms include red or pink eyelid margins, a red or pink eye, dryness, burning, blurred vision and a swollen eye.
melanin Pigment that colors the iris of the eye as well as other parts of the body, including skin and hair.
metamorphopsia Vision problem in which objects appear distorted. For example, straight lines may appear to be wavy, curved or bent, objects may appear to be larger or smaller than they actually are, or closer or farther away than they actually are. Metamorphopsia is typically caused by conditions or diseases that affect the eye's macula and retina.
microcornea Abnormally small cornea.
microkeratome Small instrument that surgeons use to cut the cornea.
microphthalmia Congenital defect resulting in an abnormally small eye or eyes. The cause is usually unknown. Microphthalmia typically results in blindness or reduced vision, but normal vision is possible if the eyes are nearly normal in size.
minification Making objects appear smaller; the opposite of magnification. Most lenses for nearsightedness make objects look smaller, and when the lenses are in eyeglasses, they also make the wearer's eyes look smaller. Aspheric lenses reduce this minification effect, for a more natural look.
mixed astigmatism Abnormal curvature of the eye's surface (cornea) that causes focusing problems at both near and distant ranges.
monochromatic Refers to one wavelength of light, as opposed to the many wavelengths of light found in varying colors.
monofocal Type of spectacle lens, intraocular lens (IOL) or contact lens design that has only one area through which the eye focuses. A multifocal lens has more than one focal area, enabling sight at multiple distances, typically for people with presbyopia.
monovision Vision correction method for those with presbyopia in which one eye is corrected for near vision and the other for far, either through contact lenses or refractive surgery. Monovision eliminates the need for reading glasses, but does have some drawbacks, including decreased depth perception.
mucin Lubricant such as saliva that protects body surfaces. In the eyes, mucin is a tear layer that helps moisten and protect the eye's surface.
mucormycosis Fungal infection typically occurring in the sinuses or lungs and mainly acquired by those with compromised immune systems and by diabetics. Symptoms include sinusitis, eye and facial pain, fever, a bulging eye and vision loss.
multifocal Type of spectacle lens, intraocular lens (IOL) or contact lens design that includes more than one area through which the eye focuses. such as bifocals or trifocals. Examples are bifocals or trifocals. This enables sight at multiple distances, typically for people with presbyopia.
myokymia Common eyelid twitch typically brought on by stress or fatigue.
myopia Also called nearsightedness. Condition in which the length of the eye is too long, causing light rays to focus in front of the retina rather than on it, resulting in blurred distance vision. Additional symptoms include eyestrain, poor night vision and squinting.
N
nanometer A measurement of length equal to one-billionth of a meter. Abbreviated as nm. Wavelengths of light are measured in nanometers. Eyeglass and sunglass lens manufacturers use nanometers to describe the different types of light that may pass through or be blocked by a lens, whether it be visible light, ultraviolet light, blue light, etc. For example, visible light has a wavelength range of 400 to 700 nm. The amount of light transmission itself is described as a percentage. For example, a dark sunglass lens might allow only 12 percent of visible light to pass through to the eye.
nearsightedness Also called myopia. Condition in which visual images come to a focus in front of the retina, resulting in defective vision of distant objects.
neovascularization Abnormal growth of new blood vessels, such as in an excessive amount, or in tissue that normally does not contain them, or of a different kind than is usual in that tissue. Also see angiogenesis.
neuronal ceroid lipofuscinosis Rare, hereditary, degenerative disease in which the body does not store pigments called lipofuscins properly; the disease is characterized by vision loss, seizures and dementia. Types include Santavuori-Haltia (infantile), Jansky-Bielschowsky (late infantile), Spielmeyer-Vogt (juvenile) and Kufs' disease (adult). Some types may also be called Batten disease.
neurons Nerve cells that form intricate networks through which sensory information is transmitted via electrical impulses sent throughout the body. Neurons are part of the central and peripheral nervous systems, which support functions such as thought, sight, perception, hearing, taste, feeling, speech and movement.
neuroretinitis Inflammation of the optic nerve and retina, commonly caused by an infection. Symptoms include blurred vision, headache, floaters, eye pain or discomfort, vision loss and loss of color vision.
nevus Birthmark, freckle or mole that is often brownish, but can be other colors as well. A nevus can occur on the skin or inside the eye and can become a melanoma, a type of cancerous growth. If your eye doctor discovers a nevus within your eye, he or she will want to check it regularly to see if it grows or becomes a melanoma that requires treatment.
nose pad One of a pair of pads, usually clear, that rest on either side of your nose and help to support your glasses.
nystagmus Rapid and involuntary eye movement that is oscillating and non-chaotic. Blurred vision may result. Nystagmus typically affects infants and has a variety of causes.
O
OD Abbreviation for "oculus dexter," the Latin term for "right eye." Or, doctor of optometry. Optometrists perform eye exams, treat diseases of the eye, prescribe medication, and write prescriptions for eyeglasses and contact lenses.
ocular herpes Recurrent viral infection that can cause inflammation and scarring of the cornea. It is not sexually transmitted. There are various types of ocular herpes, ranging from herpes keratitis to more serious forms that can lead to blindness.
ocular hypertension Condition in which the intraocular pressure of the eye is elevated above normal and which may lead to glaucoma.
ocular migraine Visual phenomena that may accompany a migraine headache or that may occur without any headache. They include light flashes, spots, wavy lines, flickers, zig-zagging lights, semi-circular or crescent-shaped visual defects and distortions of shapes.
ophthalmologist A medical doctor (MD) who specializes in the eye. Ophthalmologists perform eye exams, treat disease, prescribe medication, and perform surgery. They may also write prescriptions for eyeglasses and contact lenses.
ophthalmoplegia Eye muscle paralysis. Causes include stroke, multiple sclerosis, a tumor, thyroid disease, migraines and progressive supranuclear palsies. Symptoms can include limited eye movement, blurred vision, double vision, nystagmus and ptosis. Tolosa-Hunt Syndrome, often called "painful ophthalmoplegia," is characterized by intense pain behind the eye and a headache.
opsoclonus Rapid and involuntary eye movement that is irregular and chaotic; sometimes called "dancing eyes." Typically seen in Opsoclonus-Myoclonus Syndrome.
optic The rounded, central portion of an intraocular lens (IOL) used in cataract surgery. Optic also more generally refers to eyes or vision.
optical coherence tomography A method of imaging that, in ophthalmology, uses light waves to provide cross-sectional views of interior eye structures. Also known as OCT.
optic nerve The nerve that carries electrical impulses from photoreceptor cells (rods and cones) in the retina to the visual cortex in the brain.
optic nerve head Also called optic disk. Circular area where the optic nerve enters the retina, and the location of the eye's blind spot.
optic nerve problem The optic nerve (second cranial nerve) is the part of the eye that carries stimuli from the rods and cones to the brain. Problems such as inflammation (optic neuritis), tumors and swelling can lead to symptoms such as: blurred vision, loss of color vision, floaters, headache, eye pain or discomfort, nausea and vision loss.
optician In the United States, opticians are not doctors, but in some states they must complete training and be licensed. And in some states they can, after special training, become certified to fit contact lenses. (Please visit the Opticians Association of America website for licensing requirements for various states.) Most opticians sell and fit eyeglasses, sunglasses, and specialty eyewear that are made to an optometrist's or ophthalmologist's prescription. Many also have equipment on the premises so they can grind lenses and put them in frames without ordering from a lab.
optometrist Doctors of optometry (ODs) examine eyes for both vision and health problems, treat diseases of the eye, prescribe glasses, and fit contact lenses. They can prescribe many ophthalmic medications and may participate in your pre- and postoperative care if you have eye surgery. ODs must complete four years of post-graduate optometry school for their doctorate.
orbit Eye socket.
orbital pseudotumor An inflammatory mass in the tissues around or behind the eye that looks like and mimics the symptoms of a tumor. The cause is unknown. The primary symptom is a painful, bulging eye. You may also experience pain or discomfort around the eye.
orthokeratology (ortho-k) Procedure in which a doctor fits you with special gas permeable contact lenses to reshape your cornea and correct errors like nearsightedness. Often, patients wear the lenses just at night.
OS Abbreviation for "oculus sinister," the Latin term for "left eye."
OU Abbreviation for "oculus uterque," the Latin term for "each eye," used in vision correction prescriptions to indicate both eyes. Also an abbreviation for "oculi unitas" or "oculi uniter," meaning both eyes working simultaneously together.
overconvergence Condition in which the eyes come too far inward when focusing on a near object, resulting in blurring.
overflow tearing Common congenital condition in infants and developmental condition in older adults caused by a blocked tear duct. In infants, a membrane blocks the tear drainage duct, resulting in excessive tears and mucus. In adults, the cause of the blockage is usually unknown, but can be related to poor eyelid function.
P
papilla Small bump where the optic nerve exits the eye.
papilledema Swelling with accompanying compression of the optic nerve head, which can be a medical emergency. Causes of papilledema can include bleeding near the vicinity of the optic nerve and abnormally high cerebrospinal fluid (CSF) pressure. Autoimmune disorders, trauma and infections of the central nervous system also can cause papilledema. Optic nerve swelling can result from central retinal vein occlusion or may occur as a side effect of medications such as lithium, tetracycline and corticosteroids.
papilloma Usually benign tumor, such as a wart or a skin tag; papillomas may be raised or flat, and can be a variety of colors, such as skin-colored, yellow, pink, brown or black. Eye papillomas are typically on the eyelid, but may also appear on the conjunctiva. The cause of papillomas is felt to be viral.
Parinaud dorsal midbrain syndrome Inability to look up, typically associated with a brain lesion, characterized by nystagmus and pupil unresponsiveness to light. Causes include hydrocephalus ("water on the brain") and tumors of the pineal gland.
Parkinson's disease Neurological disorder characterized by tremors, muscle rigidity, a shuffling walk and a mask-like appearance in the face. Parkinson's may also cause infrequent blinking.
pars plana Posterior part of the eye's ciliary body.
penetrating keratoplasty A transplant procedure in which a circular area of surface eye tissue is removed from a healthy donor cornea and transferred to a recipient. A penetrating keratoplasty or corneal transplant may be needed in case of eye damage from injury or from eye diseases such as keratoconus.
peripheral vision The edges of your visual field.
phacoemulsification Also called "phako," this in-office cataract surgery procedure involves using a device with a vibrating, ultrasonic tip to break up the cataract, then suctioning the pieces out with a tiny needle.
phakic An eye that still has its natural lens. When an eye is aphakic, usually the lens has been removed during cataract or other eye surgery.
phoropter Device that provides various combinations of lenses used for tests of vision errors in eye examinations.
photochromic Able to change lens color or darkness/density depending upon the degree of exposure to light.
photocoagulation Use of heat from a high-energy laser to seal off bleeding in damaged tissue. Photocoagulation also may prevent formation of abnormal blood vessels (neovascularization) in eye diseases such as age-related macular degeneration or diabetic retinopathy. Photocoagulation also may be used to reattach a detached retina.
photokeratitis "Sunburn" of the cornea; symptoms include discomfort, blurred vision, and light sensitivity. The temporary vision loss that can result is called "snow blindness."
photophobia Discomfort from sun or other light. Photophobia has many causes.
photopsia Flashes of light often noticed in the edges of the visual field. Photopsia can have many causes, including mechanical (rather than visual) stimulation of light-sensitive cells (photoreceptors) in the retina. For example, a detached retina can cause photopsia when the retina pulls away or detaches from tissue in the inner back of the eye.
photoreceptor A light-sensitive cell found in the retina. Photoreceptors in the human retina are classified as cones and rods. Cones are located in the central retina (the fovea) and control color vision. Rods are located outside the fovea and control black/white vision in low-light conditions.
phytochemicals Chemicals found in plants that help protect against disease.
pinguecula A yellowish, thickened lesion on the conjunctiva near the cornea. Pingueculae represent a benign degenerative change in the conjunctiva caused by the leakage and deposition of certain blood proteins through the permeable capillaries near the limbus.
plano A term eye care professionals use to describe lenses with no corrective power. The term is most often applied to nonprescription sunglasses or contact lenses that are worn for cosmetic purposes only.
PMMA (polymethyl methacrylate) Old-fashioned hard contacts were made of PMMA, which is not permeable; today's rigid lenses contain other polymers that allow oxygen to reach your eye.
polarized lenses Lenses that block light reflected from horizontal surfaces such as water, to reduce glare.
polycarbonate Plastic that is very impact-resistant, and is thus sometimes used for spectacle lenses and frames.
posterior chamber Part of the eye behind the iris and in front of the lens.
presbyope Person who has difficulty reading print and seeing near objects.
presbyopia Condition in which the aging eye beginning at around age 40 is unable to focus at all distances, often noticed when print begins to blur. Additional symptoms include eyestrain, headaches, and squinting.
prescription lenses Lenses that provide vision correction as prescribed by an eye care practitioner.
prism In optics, a lens that can have precise geometric configurations enabling light to be bent or reflected in certain ways. A prism also can split white light into different wave lengths and colors.
PRK (Photorefractive Keratectomy) Surgical procedure in which an excimer laser is used to remove corneal tissue to correct vision problems.
progressive lenses Also called progressive addition lenses or PALs. Multifocal lenses whose corrective powers change progressively throughout the lens. A wearer looks through one portion of the lens for distance vision, another for intermediate vision, and a third portion for reading or close work. Each area is blended invisibly into the next, without the lines that traditional bifocals or trifocals have.
proliferative vitreoretinopathy (PVR) A common complication of surgery for retinal detachment, which causes scarring of the retina. PVR may require a vitrectomy and intricate surgical removal of scar tissue.
propionate A soft, flexible material that is sometimes used in goggles.
prosthetic Refers to a prosthesis, which is an artificial replacement for a part of the body.
protective eyewear Eyewear made with impact-resistant lenses, usually polycarbonate, that protects the eyes, especially in working situations or sports.
ptosis Drooping eyelid. Congenital ptosis is caused by a problem with the levator muscle (which lifts the eyelid). In adults, ptosis is commonly caused by the aging of the levator's connective tissue.
puncta Tiny openings through which tears drain away from the eyes. Four puncta are in the nasal corner of the eye - two in the upper inner eyelid and two in the lower inner eyelid. Punctal plugs sometimes are used to block these openings so that more tears are retained as a treatment for dry eye syndrome.
punctal cautery A procedure that uses heat or laser energy to permanently close channels from which tears drain. Punctal cautery increases the accumulation of moisture as a treatment for dry eye.
punctal plugs Tiny inserts often made of plastic that are placed in channels or ducts of the eye where moisture drainage occurs. Punctal plugs can help stop excessive drainage to keep the eye moistened in conditions such as dry eye syndrome.
pupil The round, dark center of the eye, which opens and closes to regulate the amount of light the retina receives.
pupillary distance This is the distance between the center of each pupil. Opticians use a special ruler to measure your pupillary distance before ordering your eyeglasses. It is an essential measurement because the optical center of each eyeglass lens must be positioned directly over the center of each pupil. An incorrect measurement means you would have difficulty focusing when wearing the glasses.
R
reading glasses Also called readers. Glasses to help with close work, particularly for people who are presbyopic.
refraction The test performed during an eye exam to determine the eyeglass lens powers needed for optimum visual acuity. An automated refraction uses an instrument that does not require the patient to respond. A manifest refraction is the manual way to determine the best lenses, by placing various lenses in front of the patient's eyes and asking, "Which is better, lens A or lens B?"
refractive error When light rays don't properly refract from the cornea to the retina, it is a refractive error. This can take the form of myopia (nearsightedness), hyperopia (farsightedness) or astigmatism.
refractive surgery Surgery that corrects visual acuity, with the objective of reducing or eliminating the need for glasses and contacts. Includes radial keratotomy, PRK, LASIK, and corneal implants.
replacement schedule How often you discard and replace your contact lenses: every day, week or two weeks (disposable); or every month, two months or calendar quarter (frequent replacement). It's important to differentiate between replacement schedule and wear schedule. Wear schedule is either daily wear (removed before sleeping) or extended wear (you may sleep with them in).
retina The sensory membrane that lines the back of the eye. Cells in the retina called photoreceptors transform light energy into electrical signals that are transmitted to the brain by way of the optic nerve.
retinal detachment Condition where the retina separates from the choroid. Retinal detachments have many causes, including aging, surgery, trauma, inflammation, high myopia and diseases such as diabetic retinopathy, retinopathy of prematurity and scleritis. Symptoms include light flashes, floaters, a shadow coming down over your vision, blurred vision and vision loss.
retinal tear A tear or split in the retina typically caused by a vitreous detachment. Symptoms include floaters and light flashes.
retinitis Inflammation of the retina. Symptoms include blurred vision, metamorphopsia, floaters and vision loss.
retinitis pigmentosa Usually inherited condition characterized by progressive degeneration of the retina, resulting in night blindness and decreased peripheral vision.
retinopathy of prematurity An eye disease common in premature babies that involves abnormal growth of blood vessels in the inner back of the eye (retina). Also known as ROP.
retinoschisis Condition in which the retina splits into layers, sometimes causing blurred vision. It is either inherited or acquired; the acquired form is caused by small cysts in the eye. You may also have floaters.
RGP (Rigid Gas Permeable) Type of contact lens made of breathable plastic that is custom-fit to the shape of the cornea. RGPs are the successor to old-fashioned hard lenses, which are now virtually obsolete.
rhegmatogenous Arising from a break or tear, describing a common type of retinal detachment.
RK (Radial Keratotomy) Surgical procedure where cuts are made in the cornea in a radial pattern, to flatten the cornea and correct myopia. However, RK now is virtually obsolete as a corrective eye procedure.
rod A photosensitive receptor in the retina that helps you to see in low light.
S
sclera The outer coat of the eyeball that forms the visible white of the eye and surrounds the optic nerve at the back of the eyeball.
scleritis Inflammation of the sclera. Autoimmune disorders are the most common cause. Symptoms include a red or pink eye, eye pain, light sensitivity, tearing and blurred vision.
scotoma Blind spot within the field of view.
segment A part, as in the near-vision portion of a pair of bifocals.
silicone Type of flexible and comfortable plastic. Because it is commonly used in nose pads in eyeglasses, people who are allergic to silicone should ask their eye doctor for a different type of nose pad.
single vision A lens that has the same power throughout the entire lens, in contrast to a bifocal or multifocal lens that has more than one lens power.
sinusitis Inflammation of the sinuses, due to an infection or an allergic reaction. Probably the most common cause of pain in and around the eye. Symptoms include head pain (headache, pain around the eyes, toothache, jaw pain), nasal discharge, postnasal drip, coughing, eyelid swelling, swelling around the eyes, a stuffy nose, fatigue, bad breath and a sore throat.
Snellen Chart Standard chart with letters, numbers, or symbols printed in rows of decreasing size used by eye care professionals in distance visual acuity testing. The chart was invented by Dutch ophthalmologist Hermann Snellen.
soft contact lenses Contacts made of gel-like plastic containing varying amounts of water.
solution Product used to clean, disinfect and store contact lenses.
spherical A contact lens design that is like a sphere and is fairly common; in contrast, toric lenses for astigmatism are football-shaped and are less common.
spots Small, cloudy specks in the eye that become noticeable when they fall in the line of sight.
spring hinge Type of hinge on eyeglass frames that is more flexible than a regular hinge, making the frames more durable.
stereopsis Three-dimensional vision, enabling depth perception.
strabismus A misalignment of the eyes: the eye don't point at the same object together. Crossed eyes (esotropia) are one type of strabismus; "wall-eyes" (exotropia) are another. The exact cause is unknown, but appears to be a problem with the eye muscles. Strabismus can affect depth perception.
stroma The cornea's middle layer; it consists of lamellae (collagen) and cells, and makes up most of the cornea.
sty A small red bump on the edge of the eyelid caused by an infected gland. Additional symptoms include eyelid pain, eyelid swelling, eye pain or discomfort, foreign body sensation, light sensitivity and tearing.
subconjunctival hemorrhage Bleeding from blood vessels on the surface of the eye that leaves a red patch. This common problem can be caused by sneezing, coughing, high blood pressure, trauma and more.
suspensory ligament Also called zonule of Zinn. Membrane of fibers (zonules) that holds the eye's lens in place.
T
temple The "arm" of a pair of glasses, running from the ear to the lens area.
titanium A type of metal alloy that is very strong. Eyeglasses made of titanium are lightweight, durable and often hypoallergenic.
toric A lens design with two different optical powers at right angles to each other for the correction of astigmatism.
trabecular meshwork Porous, spongy tissue within the drainage angle of the eye, through which fluids (aqueous humor) exit the eye.
trauma Injury, such as from being poked in the eye or hit in the head. Depending on the type of trauma, symptoms can include blurred vision, a bulging eye, burning, double vision, dry eyes, floaters, light sensitivity, pain or discomfort of the eye or around the eye, swelling, a pupil that is dilated or unresponsive to light, vision loss, limited eye or lid movement, ptosis, an iris defect and an eyelid cleft.
trifocal A lens design that has three focal areas: a lens for close work or reading, a lens for mid-distance viewing or arm's length, and a lens for faraway viewing or driving.
20/20 vision Many eye care practitioners consider this the average visual acuity for human beings, but humans can see as well as 20/15 or even 20/10. People with 20/40 vision can see clearly at 20 feet what people with 20/20 vision can see clearly at 40 feet. In most of the United States, 20/40 is the lowest uncorrected acuity required for a driver's license.
U
ultraviolet (UV) The invisible part of the light spectrum whose rays have wavelengths shorter than the violet end of the visible spectrum and longer than X rays. UVA and UVB light are harmful to your eyes and skin.
uvea Middle layer of the eye, below the limbus, and consisting of the iris, ciliary body and choroid.
uveitis Inflammation of the uvea. In most cases, the cause is unknown.
V
vergence disorder Vergence refers to the eyes' ability to turn either inward (convergence) or outward (divergence); convergence insufficiency is the most common vergence disorder. The disorders' exact causes are unknown. Symptoms include double vision, eyestrain, fatigue, headache, squinting and difficulty concentrating (particularly while reading).
visual acuity Sharpness of vision, usually as measured with the use of a Snellen chart. 20/20 is considered normal visual acuity, though some people can see even better (such as 20/15 or 20/10).
vitreous body Part of the eye between the lens and the retina, containing a clear jelly called the vitreous humor.
vitreous detachment Separation of the vitreous from the retina, caused by age-related vitreous shrinkage. Floaters are the typical symptom, but some people experience flashes of light as the vitreous tugs or causes traction on the retina prior to complete separation.
W
wavefront Describes technology used to "map" how the eye processes images, enabling correction through surgery or lenses for obscure vision errors.
wear schedule How long you wear your contact lenses: either daily wear (you remove the lenses each night) or extended wear (you may sleep with them in). It's important to differentiate between wear schedule and replacement schedule — that is, how often you discard and replace your lenses.
white dot syndrome One of a group of inflammatory conditions that are characterized by white dots in the retina and choroid. Symptoms can include blurred vision, loss of color vision, floaters, light sensitivity, metamorphopsia and vision loss.
wraparound Also called "wrap" for short. Type of eyeglass or sunglass frame that curves around the head, from the front to the side. Wraparound sunglasses offer extra sun and wind protection at the sides. Many cannot accept prescription lenses, because the curvature causes optical distortion. However, newer styles have been engineered to overcome this problem.
X
xanthelasma A yellow, fatty spot or bump on the inner corner of either the upper eyelid, the lower one or both eyelids, often caused by a lipid disorder such as high cholesterol.
Z
zeaxanthin A pigmented substance (carotenoid) found in yellow or orange plants, such as corn and squash, or in dark green, leafy vegetables. Zeaxanthin is being investigated for a possible association with promoting healthy vision.
zyl Zylonite, or cellulose acetate, is a lightweight plastic often used in eyeglass frames. It often appears in laminated form, with layers in different colors, but it can also be made in mottled patterns to imitate natural tortoise shell or animal skins. More about eyeglass frame materials.
