20/20: A description of normal vision. In this ratio, the top number refers to the standard distance an individual stands from an eye chart while testing vision (this number generally is fixed at 20 feet). The bottom number quantifies the distance at which an individual stands and can read the eye chart. Hence, if a person stands at 20 feet and can read the eye chart, he has 20/20 vision.
anterior chamber: The area filled with fluid that lies behind the cornea (the top layer of the eye) and the iris (the middle layer of the eye).
aqueous humor: The clear, watery fluid that generated by the ciliary body that keeps the eye adequately pressurized and provides nutrients to the eyeball. If the aqueous humor can’t properly drain from the eye, glaucoma results.
arcus: The cloudy area composed of oily or fatty compounds around the outside of the cornea. While the elderly are more likely to suffer from this condition (known as arcus senilis), those under 40 may also suffer from it (known as arcus juvenilis) usually due to high cholesterol.
artificial tears: Eye drops that you can use to lubricate eyes in the absence of natural tear production. Artificial tears alleviate redness, dryness, irritation and itchiness.
asthenopia: Also known as "eye strain," asthenopia refers to the condition in which eyes are irritated from overuse. Symptoms include redness, headaches and discomfort.
astigmatism: A condition marked by a misshaped cornea (any shape that doesn’t resemble the back of a spoon) that distorts light rays hitting the retina. Symptoms include headaches and blurry vision.
bifocals: A type of eyeglasses that has a different focal length for each lens.
binocular vision: A type of vision in which each eye perceives a slightly different scene in order to build a 3D, stereoscopic image. Humans have binocular vision, endowing us with depth perception.
blind spot: The area within the field of vision in which a person is sightless due to the lack of photosensitive cells. Blind spots occur where the optic nerve meets the eye.
cataract: The condition in which a cloudiness on the crystalline lens prevents the retina from perceiving a clear, accurate image. If a cataract worsens, a patient may need to get it surgically removed. Causes of cataracts include smoking, diabetes, UV rays and using steroids.
color blindness: The condition in which an individual can’t distinguish between different colors, generally red and green. While color blindness is generally caused by genetic defect, it may also result from damage to the eye, optic nerve or brain. Men suffer from color blindness at higher rates than women.
computer vision syndrome: A condition marked by a set of symptoms associated with extended starring at a computer screen. Symptoms include irritation, headaches, redness, blurred vision and sensitivity.
conductive keratoplasty (CK): An optical procedure that uses radiofrequencies to treat farsightedness (hyperopia).
cone: Photosensitive cells in the retina that detect color in relatively bright light. The retina contains about 6 million cones.
conjunctivitis: Commonly known as "pink eye," conjunctivitis is a condition that arises out of an allergic reaction or an infection. Symptoms of conjunctivitis include redness, dryness, irritation and, at times, itchiness.
cornea: The transparent, outer layer of the eye that protects the inner layers while also allowing light to enter.
corneal transplant: More formally known as a keratoplasty, a corneal transplant is a surgical procedure that replaces a diseased part of the cornea with a healthy clear cornea that usually comes from a deceased organ donor.
corrective lens: An artificial lens placed on top of the cornea to correct a variety of vision problems, including myopia, astigmatism and presbyopia.
daily wear contact lenses: Contact lenses that can only be worn during the daytime (or periods less than 24 hours). Daily wear contact lenses must be disinfected in saline solution after each wear.
diabetic retinopathy: An optical condition that results as a side effect of advanced or long-term diabetes. Diabetic retinopathy occurs when the small blood vessels in the retina swell and leak. Eight percent of the cases of diabetic retinopathy are treated in their early stages.
diopter: A measurement optometrists use to prescribe glasses and contacts. While a negative diopter number describes prescriptions for nearsighted patients, positive diopter numbers describe prescriptions for the farsighted.
diplopia: Commonly known as "double vision," diplopia is a condition in which an individual perceives two images from a single object on either a vertical, horizontal or diagonal plane.
disposable contact lenses: In strict terms, disposable contacts are a type of lens that should be thrown away after a single use. In practice, these lenses are worn anywhere from one to 14 days before needing to be replaced.
dry eye: The condition in which the eye suffers from a lack of natural moisture. Artificial tears, also known as eye drops, are the most common way to treat dry eyes.
endothelium: Cells that layer and line blood vessels, specifically those in the cornea.
epiphora: The production of tears or other eye discharge that overflows onto the cheeks.
esotropia: Commonly known as "cross eyes," estropia is the condition in which one eye is misaligned, turning inwards towards the nose, while the other eye is normal.
exotropia: As the condition that is the opposite of esotropia, exotropia refers to the condition in which one eye is turned outward while the other is normal. Exotropia is commonly known as walleye.
extended wear contact lenses: A type of contact lens made to be worn 24 hours per day for up to seven days on end. At the end of this 7-day period, the wearer must insert a fresh pair of contacts.
extraocular muscles: A group of six muscles that allows the eye to move around in its socket. The muscles that make up this group include the lateral rectus, the superior rectus, the inferior rectus, the medial rectus, the superior oblique and the inferior oblique.
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