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“You cannot depend on your eyes when your imagination is out of focus.” — Mark Twain

Glossary of Terms


Conjunctiva A tissue-paper thin membrane that is on the surface of the white part of the eye. It contains many tiny blood vessels which dilate when irritated and cause the eye to be red (see pink eye).
Cornea The clear front surface of the eye.
Iris The colored part of the eye, usually blue, brown or hazel.
Lens The part of the eye that changes the focus of the eye by changing its shape.
Optic Nerve The nerve that carries information from the retina to the brain.
Pupil An opening in the iris that can change size depending on lighting conditions.
Retina The lining of the eye which, like film in a camera or a movie screen, records the image you see.
Sclera The white outer shell of the eye seen beneath the thin conjunctiva. It is a tough protective layer of tissue.
Visual Field The entire area that is seen, including central vision (for reading) and peripheral or side vision.
Vitreous A jelly-like substance that fills the inside of the eye.


Astigmatism A common finding relating to the shape of the cornea (the front surface of the eye). The cornea's curve vertically is different than its curve horizontally, like the bowl of a spoon.
Farsightedness (Hyperopia) The ability to see distant and near objects with changes in focusing of the lens; a common condition that usually does not require glasses until a person is in their 40's or 50's.
Nearsightedness (Myopia) The ability to see near objects clearly with or without glasses, but distant objects are blurred without glasses.
Presbyopia Literally "elder vision", this is a gradual loss of the ability to focus on near objects. This is a normal change that happens to everyone and it is corrected by reading glasses or bifocals.


Amblyopia or Lazy Eye Poor vision in one eye due to 1) abnormal alignment of the two eyes, or 2) a significant difference in the refraction (need for glasses) in the two eyes, or 3) blockage of a good image from cataract, corneal scar, droopy eye lid, etc.
Cataract A haze or clouding of the lens of the eye due to normal aging changes. Cataracts blur vision but do no permanent damage to the eyes.
Detached Retina A problem in which the inner lining of the eye (the retina) separates from the back of the eye, usually accompanied by flashing lights, floaters and blurring of vision or loss of side vision.
Diabetic Retinopathy Abnormal blood vessels formed in response to chronically abnormal blood sugar. These blood vessels tend to break and cause bleeding, swelling and scar tissue in the retina.
Esotropia A condition in which one eye turns in as if looking toward the nose, usually due to a weak eye muscle, poor vision in one eye, or abnormal innervation to the eye muscles.
Exotropia An eye condition in which one eye turns out due to a weak eye muscle, abnormal innervation to the eye muscle, or lack of coordination of eye movements.
Floaters Small dots, irregular lines, blobs, or spider web appearing objects that float or wander across your visual field; these are usually due to small pieces of vitreous gel or blood in the eye. They are usually not a problem, but may rarely provide warning of a retinal detachment.
Glaucoma An eye condition in which the main nerve of the eye (the optic nerve) is damaged due either to intolerance of the eye pressure and/or to poor circulation around the nerve.
Iritis An irritation of the colored part of the eye that causes redness, pain and light sensitivity.
Keratoconus A condition in which the cornea is thin and bulges forward irregularly causing poor vision.
Lazy Eye A term used for a poor vision eye (as noted under amblyopia), for an eye that wanders in or out, or for an eye that has a droopy eye lid.
Macular degeneration Decrease or loss of reading vision due to damage to the central retina from abnormal circulation and/or nutritional factors.
Nystagmus Irregular, uncontrollable jerking or wobbling movements of the eyes usually seen in very poor vision eyes.
Optic Atrophy A degeneration of the main nerve of the eye that carries the image to the brain.
Pink Eye A red color of the lining of the eye that is due to infection (bacterial or viral) inflammation (allergy, dryness, pollution or smoke) or bleeding (broken blood vessels).
Posterior Capsular Opacity A film that may develop behind the intraocular lens, that may cause blurring of vision; this film may form months to years after cataract surgery.
Pterygium A thickened or fleshy tissue on the white part of the eye that grows onto the cornea covering the colored part of the eye (the iris).
Ptosis A droopy eye lid due to weakness of the muscle that lifts the lid or to excess skin that covers the eye; this may be in one or both eyes.
Sty A localized infection of an oil gland in the eyelid, like a pimple.
Sub-conjunctival Hemorrhage A red spot on the white of the eye due to a broken blood vessel; it is like a bruise and may vary in size.

Optical Terms

Bifocal Glasses with lenses that focus at two different distances; near for reading and far for driving or watching TV.
Progressive Bifocal "No line" bifocals that focus at multiple distances.
Refraction The process of determining the lens power that will correct an eye that is nearsighted, farsighted or has astigmatism and allow that eye to see its best.
Trifocal Glasses with lenses that focus at three different distances; near for reading, far for driving, and at arm's length for computer use, piano playing, etc.


Intraocular Lens A plastic lens (similar in shape to a contact lens, but smaller) that is placed in the eye after cataract surgery to correct the patient's vision.