The Glossary

Understand all those fancy words you are reading on the site.

Glossary of Terms

Accommodative disorder

Accommodation refers to the eye’s ability to automatically change focus from seeing at a distance to seeing at near. Accommodative disorders have a variety of causes. Symptoms include blurred vision, double vision, eye strain, headache, fatigue and difficulty concentrating (particularly while reading). Presbyopia is an accommodative disorder that eventually affects almost everybody, since its causes relate to the aging of the eyes.

Allergy

Reaction of the body’s immune system to a foreign substance (e.g., pollen, animal dander). When the eyes are affected, the most common symptoms are redness, itching, chemosis, tearing, swollen eyelids and stickiness. 

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. There are no symptoms. The patient may be found squinting and closing one eye to see; there may be unrecognized blurred vision in one eye and vision loss. 

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).  

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.

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.

ANTI-FOG LENSES​

Lenses specifically designed to resist fogging in everyday conditions, providing clearer vision for the wearer.

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. 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.

ASTIGMATISM​

A refractive error where objects at all distances have a distorted appearance (“streaked” or stretched, with elongated shapes and unequally blurred borders). Symptoms of astigmatism include squinting, eye strain, headaches and reading problems.

aura

A sensation experienced before an attack of epileptic seizure, migraine or other disorder. Examples are flashes of light, colored lights, numbness, coldness and even hearing voices.

BEST CORRECTED VISUAL ACUITY (BCVA)​

(Best Corrected Visual Acuity) The best possible 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.

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.

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. 

BLUE LIGHT​

Found in the light spectrum next to violet, which has the shortest wavelength of visible light. Also known as High Energy Visible (HEV) light, blue light is emitted from the screens of smartphones, computers, and even certain TVs. It has been linked to eye fatigue and possibly eye damage.

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.

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​

A collection of problems, mostly eye- and vision-related, associated with computer use. Also known as digital eye strain, symptoms can include dryness, blurred vision, red or pink eyes, burning, light sensitivity, headaches and pain in the shoulders, neck and back. 

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 PROBLEM ​

Contact lens problems can range from minor to sight-threatening, and include protein buildup, 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.

CONVERGENCE ​

Ability of the eyes to turn inward. People with convergence insufficiency have trouble (eye strain, blurred vision, etc.) with near tasks such as reading.

CORNEA​

The clear portion of the front surface of the eye that allows light to enter the eye for sight. The cornea provides most of the focusing power of the eye.

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 ARCUS​

A visible white or gray clouding of the peripheral cornea, caused by lipid (fat) deposits. The opacity can be arc-shaped, or it can form a complete ring around the cornea. Corneal arcus commonly seen in older adults is sometimes called arcus senilis. Corneal arcus seen in younger individuals is associated with high cholesterol.

CORNEAL DYSTROPHY​

One of a group of conditions, usually hereditary, in which the cornea loses its transparency and 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 ECTASIA​

A progressive thinning of the middle layer of the cornea (stroma), causing the front surface of the eye to bulge forward. Ectasia causes irregular astigmatism, distorted vision and corneal scarring. Causes of corneal ectasia include keratoconus, other corneal diseases, and in rare occasions LASIK surgery.

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 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 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 (trochlear), 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, 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 symptoms such 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 eyes. 

crossed eyes

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

DERMATOCHALASIS ​

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

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 may also suffer vision loss. 

diplopia

Also called double vision. When two images of the same object are perceived by one or both eyes.

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.

DOUBLE VISION ​

Also called diplopia. When two images of the same object are perceived by one or both eyes.  

DROOPING EYELIDS​

Also called ptosis. Condition in which the upper eyelid(s) sag. It can be present at birth or caused by a later problem with the muscles lifting the eyelid, called levators. 

DRUSE​

A 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 requires 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. 

ectropion

Abnormal turning out of an eyelid, typically the lower one, causing exposure of 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.

EMMETROPIA ​

The refractive condition where a person can see distant objects clearly when the eyes are at rest (no focusing effort required) and needs to exert only the normal amount of focusing effort (determined by the distance of an object from the eye) to see near objects clearly. An emmetropic eye has no nearsightedness, farsightedness or astigmatism.

ENTROPION ​

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.

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.

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.

ESOTROPIA ​

When one or both eyes point inward, appearing “crossed.” This is a type of strabismus.

EXOTROPIA ​

When one or both eyes point outward; also called “walleyed.” This is a type of strabismus.

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 known as hyperopia.

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, experiencing a “shower of floaters” can signal 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.

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 eye care practitioners, frequent replacement lenses are discarded monthly or quarterly, whereas disposable lens usage ranges from one day to two weeks.

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.  

glaucoma

Disease 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, 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. 

hard contact lenses

Rarely worn anymore, these are the small, hard lenses made of PMMA material that many people wore in the ’70s and ’80s. Compared with modern soft lenses 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.

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. Because it is denser, the same amount of visual correction occurs with less material (whether glass or plastic), allowing for a thinner lens. 

hyperopia

A refractive error where a person must exert a greater-than-normal focusing effort to see distant and near objects clearly. Depending on the farsighted person’s age and degree of hyperopia, objects may be clear or blurred without glasses. Other symptoms of hyperopia include headaches, eye strain, and avoidance of reading and other near tasks. Also called farsightedness.

intermediate vision

Generally refers to eyesight at approximately arm’s length, used for tasks such as computer work and viewing the speedometer in a car. 

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

The pigmented structure that surrounds the pupil and determines eye color. The iris also acts as a diaphragm that increases and decreases the size of the pupil to control the amount of light entering the eye. 

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

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 fails to properly improve with glasses (contacts usually work well for a while). You may also have double vision or distorted vision.

LASIK (Laser-Assisted In Situ Keratomileusis)

Surgical procedure in which a tiny flap is cut into the top of the cornea, underlying corneal tissue is removed with an excimer laser, and the flap is put back in place. 

legal blindness

A severe visual disability where either: a) the best attainable visual acuity in a person’s better eye (with corrective lenses) is 20/200 or worse on a standardized eye chart; or b) a person’s peripheral vision is restricted to no greater than 20 degrees at its widest diameter, as determined by visual field testing. 

lens

Can refer to either: a) the nearly spherical body in the eye, located behind the iris, that focuses light rays onto the retina; b) 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.

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.  

lutein

An antioxidant that is found throughout the body but is concentrated in the macula. Lutein is believed to help protect the eyes from free radical damage caused by the sun’s harmful rays.

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 left untreated.

macular hole

A 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.

meibomian gland

Gland found in the eyelid that produces the oily outer layer of the three-layer tear film that lubricates the eye.

metamorphopsia

Vision problem in which objects appear distorted. For example, straight lines may appear wavy, curved or bent, or objects may appear to be larger or smaller than they 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.

migraine

Severe headache, sometimes accompanied by nausea and visual disturbances. Visual disturbances alone are also possible; this problem is called an ophthalmic migraine, or migraine without headache. Eye and vision symptoms include blurred vision, ptosis, halos around lights, light flashes, light sensitivity, eye pain or discomfort, vision loss (blind spots in central vision, tunnel vision, or overall impaired vision), distorted vision and wavy lines in vision.

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. 

multifocal

Type of spectacle, intraocular, or contact lens design that includes more than one area through which the eye focuses. 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. A refractive error where a person can see near objects clearly, but distant vision is blurred. Myopia typically begins in childhood and progresses throughout the school years. The primary symptom of nearsightedness is squinting.  

nearsightedness

See definition for myopia.

near vision

Eyesight used for reading and other up close tasks, generally at a range of 12 to 16 inches from the eyes. 

neovascularization

Abnormal growth of new blood vessels, be it excessive or in tissue that does not normally contain them, or of a different kind than is usual in that tissue. 

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, they will want to check it regularly to see if it grows or becomes a melanoma that requires treatment.

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. 

OD

Abbreviation for oculus dexter, the Latin term for “right eye.” Or, Doctor of Optometry.

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.

optic nerve

The nerve that carries electrical impulses from photoreceptor cells (rods and cones) in the retina of the eye 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.

optician

Person who sells and fits eyeglasses, sunglasses, and specialty eyewear that are made to a prescription by an optometrist or ophthalmologist. Many also have equipment and knowledge to make the lenses that are put into the glasses. In some states they must complete training and be licensed, and in some places they may become certified to fit contact lenses after special training.

optometrist

Also known as Doctors of Optometry. Health care professionals who provide primary eyecare through comprehensive eye examinations to detect and treat various visual abnormalities and eye diseases. They can prescribe glasses, contact lenses, and medications to treat eye infections, eye conditions like glaucoma, and injuries. Optometrists typically work closely together with other eye care professionals such as ophthalmologists and opticians in a number of professional settings to deliver quality eye care to the general public.

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 together simultaneously.

peripheral vision

The edges of your visual field.

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.

photochromic

Able to change lens color or darkness/density depending upon the degree of exposure to light.

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 stimulation of photoreceptors in the retina (as opposed to visual stimulation). For example, a retinal detachment can cause photopsia when the retina pulls away or detaches from tissue in the inner back of the eye. A vitreous detachment with accompanying photopsia can occur when the eye’s gel-like interior begins to shrink and pull against the retina. Photopsia can be accompanied by a shower of floaters.

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 used by eye care professionals 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.

polarized lenses

Lenses that block light reflected from horizontal surfaces such as water in order to reduce glare. 

polycarbonate

Highly impact-resistant plastic sometimes used for spectacle lenses and frames.

presbyopia

Condition in which the aging eye (beginning at around 40) is unable to focus at all distances, often noticed when print begins to blur. Additional symptoms include eye strain, headaches, and squinting.

prescription lenses

Lenses that provide vision correction as prescribed by an eye care practitioner.

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 seamlessly into the next, without the lines that traditional bifocals or trifocals have.

protective eyewear

Eyewear made with impact-resistant lenses, usually polycarbonate, that protects the eyes, especially in working situations or sports.

pterygium

Triangular fold of tissue on the white of the eye that can eventually grow over part of the cornea. The cause may be irritation from the sun’s UV rays, dust and wind. Some people exhibit no symptoms, while others may have redness or blurred vision. Pterygia that are chronically inflamed can become itchy. 

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 aging of the levator’s connective tissue.

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 in order 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. This measurement is essential, 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.

reading glasses

Also called readers. Glasses to help with seeing close up, 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

Any of several size- and shape-related abnormalities of the eyeball (or components of the eye) that affect the eye’s normal ability to focus light on the retina.  

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.

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 then 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.

rewetting drops

Eye drops designed to re-moisten and lubricate contact lenses in order to increase comfort while they are being worn

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 mostly obsolete now.

RK (Radial Keratotomy)

Surgical procedure in which cuts are made in the cornea in a radial pattern to flatten the cornea and correct myopia. However, RK is now virtually obsolete as a corrective eye procedure.

rosacea

Skin condition that typically involves the face, characterized by flushing, red bumps and telangiectasia (dilated, visible capillaries). It is most common among fair-skinned women, who develop it between ages 30-50. 

scotoma

Blind spot within the field of view.

silicone

Type of flexible and comfortable plastic. Because it is commonly used in the nose pads of 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, in contrast to a bifocal or multifocal lens, which have 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.

Sjogren's syndrome

An inflammatory autoimmune disorder characterized by dry mouth and dry eyes. Additional eye symptoms include burning, discharge, foreign body sensation, itching and light sensitivity. 

Snellen chart

Standard eye 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.

stereopsis Three-dimensional vision, enabling depth perception.

strabismus

A misalignment of the eyes where the eyes don’t point at the same object together. Crossed eyes (esotropia) are one type of strabismus; “walleyes” (exotropia) are another. While the exact cause is unknown, it appears to be a problem with the eye muscles. Strabismus can affect depth perception. 

stye

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.  

toric

A lens design with two different optical powers at right angles to each other for the correction of astigmatism.

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 in 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.

trichiasis

Condition in which the eyelashes grow inwardly toward the eye.

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.

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.

uveitis

Inflammation of the uvea. The cause is unknown in most cases, but infectious or immunological systemic disorders can cause uveitis. Symptoms vary depending on where in the uvea the inflammation occurs. They include mild to strong eye pain, redness, light sensitivity, blurred vision and floaters. You may also experience tearing, a pupil that responds poorly to light or squinting. Specific types of uveitis include iritis, iridocyclitis, cyclitis, pars planitis and choroiditis. 

vergence disorder

Vergence refers to the ability of the eyes to turn either inward (convergence) or outward (divergence). Convergence insufficiency is the most common vergence disorder. The exact causes of these disorders are unknown. Symptoms include double vision, eye strain, fatigue, headache, squinting and difficulty concentrating (particularly while reading).

visual acuity

Sharpness of vision, usually as measured with the use of a Snellen eye chart. 20/20 is considered normal visual acuity, though some people can see even better (such as 20/15 or 20/10).

vitreous body

The part of the eye between the lens and the retina that contains 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.

vitreous hemorrhage

Bleeding that enters the vitreous from nearby parts of the eye, such as from leaking retinal blood vessels. Causes include diabetic retinopathy, trauma, a retinal tear or detachment, vitreous detachment and retinal vascular occlusion (blockage in the retina’s vascular system). Symptoms include sudden blurring or loss of vision, and new floaters.

wear schedule

How long you should wear your contact lenses for. Wear schedules can be either daily (you remove the lenses each night) or extended (you may sleep with them in). It is important to differentiate wear schedule from replacement schedule, which refers to how often you discard and replace your lenses.

xanthelasma

A yellow, fatty spot or bump on the inner corner of either the upper eyelid, the lower one or both, often caused by a lipid disorder such as high cholesterol.

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 researched for a possible association with promoting healthy vision. 

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