angiogenesis The formation of new blood vessels in the body. Also see neovascularization.

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.

aniridia Absent or partially absent iris, typically congenital. Additional symptoms include poor vision and photophobia.

anisocoria Unequal pupil size. Causes include glaucoma, head or eye trauma, an intracranial tumor, infection of the membranes surrounding the brain and previous intraocular surgery. A small percent of the population has unequal-sized pupils naturally (without any known cause).

anisometropia Condition where the eyes have a significantly different refractive power from each other, so the prescription required for good vision will be different for each eye.

anophthalmos Absence of one or both eyes. Anophthalmos may be congenital or due to trauma, infection or other causes. Symptoms include reduced depth perception and peripheral vision.

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.

antioxidant Substance that inhibits oxidation and can guard the body from the 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. It has been theorized that antioxidants can help prevent macular degeneration and other serious eye diseases; many studies are being conducted in this area. Read more about nutrition and the eyes.

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. Read more about anti-reflective coatings.

apheresis A process in which blood is drawn outside the body, certain compounds are removed, and the blood is returned to the body. The technique has various applications, including: harvesting of needed components such as plasma or white blood cells; and removing harmful components such as large proteins, in order to treat the dry form of age-related macular degeneration (AMD). For more information, please see our article on age-related macular degeneration.

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.

arcus An opaque arc or ring around the peripheral cornea, this represents fatty or oily deposits in the cornea. It is usually seen in elderly people and is called arcus senilis. Arcus juvenilis is seen in people younger than 40 and often indicates high levels of cholesterol in the blood.

Argyll Robertson pupil Small, irregular pupil, usually caused by syphilis. Argyll Robertson pupils do not respond to light.

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.

asteroid hyalosis Benign condition in which flecks of calcium-rich fats become suspended in the eye's vitreous. The exact cause is unknown, but it may be due to aging collagen in the vitreous or decomposition of hyaluronic acid in the vitreous. Asteroid hyalosis typically is without symptoms, but some people experience blurred vision or floaters.

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. Read more about astigmatism.

atopy Type of allergy where levels of the antibody immunoglobin E are increased; atopy includes rhinitis, asthma, hay fever and eczema.

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.

band keratopathy Opacity of the corneal stroma and Bowman's membrane. Symptoms include vision loss and foreign body sensation.

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 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 can apply to both eyeglass lenses and contact lenses.

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.

blepharochalasis Excessive, drooping eyelid skin caused by recurring swelling. Blepharochalasis typically occurs in young people.

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.

blepharospasm Involuntary increased blinking that progresses to spasms in both eyes. The exact cause is unknown, but doctors believe it to be a central nervous system disorder. It can produce a functional blindness since the patient can't open his or her eyes long enough to function visually.

botulism Serious illness from a toxin produced by Clostridium bacteria (usually Clostridium botulinum). Infant botulism and food-borne botulism are the most common forms in the United States. Symptoms include double vision, blurred vision, ptosis, muscle weakness, difficulty speaking, difficulty swallowing, difficulty breathing and nausea.

Bowman's membrane Corneal layer between the epithelium and the stroma.

bridge The part of eyeglasses that extends across the nose.

cable temple Style of eyeglasses that wraps around the ear, to keep them well-fastened.

canaliculitis Inflammation of a tear duct (or ducts), caused by a bacterial infection. Symptoms of this disorder include discharge, a red or pink eye and a swollen upper or lower eyelid near the nose.

carotenoid A pigmented substance that adds color such as red, orange, or yellow to plants. Carotenoids have antioxidant properties that protect cells against damage from free radicals, unstable atoms that can interact with and break down healthy tissue in different parts of the body including eyes.

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. Read more about cataracts and cataract surgery.

cavernous sinus problem The cavernous sinus is located at the base of the cranium and contains the carotid artery and cranial nerves. Problems in the cavernous sinus include tumors, aneurysms and clots. Typical symptoms include ophthalmoplegia, chemosis and a bulging eye. You may also experience a red eye and vision loss.

cellulitis Inflammation of tissue around the eye. Pre-septal cellulitis affects the lid and other "outer" areas of the eye, whereas orbital cellulitis affects the "inner" areas around the eyeball. Pre-septal cellulitis symptoms include a red, swollen lid, swelling around the eyes and eye or lid pain or discomfort. Orbital cellulitis symptoms include a bulging eye and ophthalmoplegia, as well as a red, swollen lid, swelling around the eyes, eye or lid pain or discomfort and a decrease in vision. An orbital cellulitis is an ocular emergency.

central island Refractive surgery complication in which the laser leaves an "island" of corneal tissue in the concave ablation zone. Symptoms include double vision and distortion. Read more about potential complications of LASIK and other kinds of refractive surgery.

central serous retinopathy Disorder in which fluid collects under the central retina (macular area) and disrupts central vision. The cause is unknown. Symptoms include blurred central vision and metamorphopsia. Some patients also develop floaters.

cerebral cortex Outer portion of the brain where complex functions including certain vision processes take place.

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. Read more about chalazia.

chemosis Conjunctival swelling that is often caused by an allergy.

choroid Layers of blood vessels located between the sclera (white of the eye) and the retina; they provide nourishment to the back area of the eye.

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. Read more about CK.

clip-on Type of glasses that attaches to your regular glasses, such as clip-on sunglasses.

CMV retinitis (cytomegalovirus retinitis) Serious eye infection usually found in those with immune problems, such as AIDS patients; symptoms include floaters, blind spots, blurry vision and vision loss.

collagen Fibrous protein in bones and connective tissue, it is also present in the eye. One type of vision correction surgery heats collagen around the edges of the cornea (which lets light into the eye). This procedure reshapes the cornea, helping it focus light right onto the retina, for clearer vision.

coloboma Cleft, usually due to incomplete embryologic development in utero. An iris coloboma is the most common eye coloboma; the pupil will often look like a keyhole or upside-down pear. Colobomas can also affect other eye structures, such as the eyelid, retina and optic nerve; only iris and eyelid colobomas are visible with the naked eye. Additional symptoms such as poor vision may occur, but are not readily apparent from a parent's perspective.

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 Mucous membrane that lines the visible part of the eye and the inner surface of the eyelid.

conjunctivitis Inflammation of the conjunctiva, characterized by a pink eye. The cause is either infectious or allergic; the term "pink eye" really refers to the viral variety, but 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.

cranial nerve One of the 12 pairs of nerves that go from the brain to other parts of the head. Those that affect the eyes and vision are the second cranial nerve (optic nerve), third (oculomotor), fourth (troclear), sixth (abducens) and seventh (facial). The optic nerve carries stimuli from the rods and cones to the brain. The third, fourth and sixth cranial nerves work with the eye muscles to control eye movement. The seventh cranial nerve works with the facial muscles to control facial movement (specifically the closure of the eyelids).

cranial nerve palsy Palsy (full or partial paralysis) of the third, fourth or sixth cranial nerves can result in difficulty moving the eye with such symptoms as eyes that don't point in the same direction, reduced depth perception, double vision, ptosis, vision loss, a dilated pupil that doesn't respond to light and head tilting. Causes include head trauma, diabetes, tumors, aneurysms, infarction (tissue death) and more.

In most cases, the cause of paralysis of the seventh cranial nerve is unknown (termed "Bell's palsy"). Symptoms include weak facial muscles, difficulty closing the eye, infrequent blinking, earache, acute hearing, facial drooping, ectropion, tearing, eye dryness, blurred vision and a burning feeling in the eye.

crossed eyes Type of strabismus (a misalignment of the eyes) where one or both eyes point inward, toward the nose.

cystoid macular edema (CME) Swelling of the eye's macula, caused by an excessive amount of fluid.

dacryoadenitis Inflammation of the tear gland, typically caused by a viral or bacterial infection. Symptoms include a dry eye, a red or pink eyelid, swelling of the lid or around the eyes and ptosis.

dacryocystitis Inflammation of the nasolacrimal (tear) sac, typically caused by dacryostenosis. Symptoms include discharge, a sticky eye, eye pain or discomfort, a red or pink eye, swelling around the eye and tearing.

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.

dermatochalasis Excessive, drooping eyelid skin caused by a loss of elasticity in aging skin.

Descemet's membrane Corneal layer between the stroma and the endothelium.

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.

diopter Unit which measures the refractive (light-bending) power of a lens; eyecare practitioners use it in eyeglass and contact lens prescriptions. A negative number refers to nearsightedness, while a positive number refers to farsightedness. For example, someone with -8.00 diopter lenses is very nearsighted, while someone with +0.75 diopter lenses is only slightly farsighted.

diplopia Also called double vision. When two images of the same object are perceived by one or both eyes. Please click to the Eye Symptoms A-Z page for a list of the usual causes of diplop

disposable contact lenses Technically, this is any contact lens that is thrown away after a short period of time. Among most eyecare 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. Please click to the Eye Symptoms A-Z page for a list of the usual causes of seeing double.

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.

drugs Many drugs, both legal and illegal, can affect your eyes and vision. These include eyedrops, other topical eye medications, pills and more. Symptoms can include blurred vision, burning, dry eyes, eyelash loss, floaters, halos around lights, light sensitivity, pupils that are dilated, small or unresponsive to light, peripheral or general vision loss and jaundice.

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 eyecare 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.

ectropion An abnormal turning out of an eyelid, typically the lower one, which exposes the inner, conjunctival side of the eyelid; usually due to aging. Additional symptoms include eye or lid pain or discomfort, a red or pink eye or eyelid and overflow tearing.

edema Accumulation of an excessive amount of watery fluid, which causes swelling.

emmetropia The condition of an eye with normal vision, meaning that light rays correctly are focused at the inner back of the eye (retina) where images are processed.

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.

enophthalmos The sinking of the eye into the socket. Causes include development problems in utero, trauma and inflammation.

entropion An abnormal turning in of an eyelid, which causes the lashes to rub on the ocular surface; usually due to aging. Additional symptoms include eye or lid pain or discomfort, foreign body sensation, a red or pink eye, itching, tearing and vision loss.

environmental condition Air pollution, wind and bright light can irritate your eyes and cause symptoms such as burning, dryness and tearing.

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.

excimer laser An instrument that uses shorter wave (ultraviolet) light to vaporize and remove tissue from the eye's surface during vision correction procedures.

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."

eyecare practitioner Optometrists (O.D.s) and ophthalmologists (M.D.s) both practice eyecare, but in different ways: O.D.s (Doctors of Optometry) examine eyes for both vision and health problems, prescribe glasses, prescribe and fit contact lenses, and treat some eye conditions and diseases. M.D.s are medical doctors who examine eyes, prescribe glasses and contacts, treat disease and perform surgery. Other non-doctor eyecare practitioners include paraoptometrics, contact lens technicians and opticians.

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.

farsightedness Also called hyperopia. To farsighted people, near objects are blurry, but far objects are in focus.

FDA (Food & Drug Administration) A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the U.S., these products must be approved by the FDA before they can be marketed.

flap and zap Slang for LASIK.

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.

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.

frequent replacement contact lenses Also called planned replacement. Technically, this is any contact lens that is thrown away after a moderately short period of time. Among most eyecare practitioners, "disposable" usage ranges from one day to two weeks, while "frequent replacement" lenses are discarded monthly or quarterly.

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. For more information, please see our fungal keratitis eye infection article.

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.

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.

hemifacial spasm Involuntary muscles twitches on one side of the face, typically caused by compression of the seventh (facial) cranial nerve by a neighboring blood vessel somewhere in the brain.

heterochromia Condition where one eye is a different color from the other, or one eye is more than one color.

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.

histiocytosis Abnormal proliferation of histiocytes (immune system cells). Common symptoms include bone tumors and skin rashes. If histiocytosis affects the eyes, it causes bulging.

Horner's syndrome Condition characterized by a small pupil, ptosis and an abnormal lack of facial perspiration (all on the same side of the face); Horner's syndrome is caused by injury to the sympathetic nerves of the face.

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.

hypotony Low intraocular pressure, often caused by eye surgery or trauma (e.g., open globe injury). Symptoms include blurred vision and eye pain or discomfort.