The Glossary
The words your eye doctor uses.
Short, plain-English definitions for the terms you are most likely to meet on a prescription, in a diagnosis or in an eye-health article. Each definition is drawn from the National Eye Institute or the American Academy of Ophthalmology.
- Accommodation
- The eye's ability to change focus from far to near. You lose it slowly with age — the cause of presbyopia.
- Age-related macular degeneration (AMD)
- Damage to the central part of the retina (the macula) that blurs the middle of your vision. Dry AMD is the common, slow form; wet AMD is less common but can move fast.
- Amblyopia (lazy eye)
- Vision in one eye fails to develop in childhood because the brain favors the other eye. Treated earlier is better.
- Anterior chamber
- The fluid-filled space at the front of the eye, between the cornea and the iris.
- Aqueous humor
- The clear fluid in the front of the eye. Its drainage is what glaucoma pressure medicines target.
- Astigmatism
- The cornea or lens is shaped more like a rugby ball than a football, so light focuses to more than one point. Corrected with glasses, contacts or surgery.
- Blepharitis
- Inflammation of the eyelid margins. Common, long-running, treated with warm compresses and lid hygiene.
- Cataract
- Clouding of the eye's natural lens. Common with age; the lens is replaced in a routine outpatient operation.
- Chalazion
- A firm, painless lump in the eyelid from a blocked oil gland. Often resolves with warm compresses.
- Choroid
- The layer of blood vessels between the retina and the sclera that feeds the back of the eye.
- Conjunctiva
- The thin clear tissue that covers the white of the eye and the inside of the eyelids.
- Conjunctivitis (pink eye)
- Inflammation of the conjunctiva, often from a virus, bacteria or allergy.
- Cornea
- The clear front window of the eye. It does most of the eye's focusing.
- Diabetic retinopathy
- Damage to the blood vessels of the retina from long-standing diabetes. The main reason adults with diabetes need yearly dilated eye exams.
- Dilated eye exam
- An exam in which drops widen your pupils so the doctor can see the retina and optic nerve clearly. Vision stays blurry and light-sensitive for a few hours after.
- Diopter (D)
- The unit of a lens prescription's strength. The number on a glasses prescription.
- Diplopia
- Double vision — seeing two of one object. New diplopia should be seen by a doctor the same day.
- Dry eye
- A condition where the eye does not make enough tears, or the tears evaporate too fast, causing dryness, grit and sometimes blurry vision.
- Emmetropia
- The eye's natural, no-prescription state — light focuses exactly on the retina.
- Floaters
- Small shapes that drift across your vision, from debris in the vitreous. Occasional floaters are normal. A sudden shower with flashes is not — see a doctor the same day.
- Fovea
- The tiny central pit in the macula that gives you the sharpest vision.
- Glaucoma
- A group of conditions that damage the optic nerve, most often linked to pressure inside the eye. It can cause slow vision loss with no early warning.
- Hyperopia (farsightedness)
- Close objects are harder to focus than far ones. Marked with "+" on a prescription.
- Intraocular pressure (IOP)
- The pressure of fluid inside the eye. High IOP is a major risk factor for glaucoma.
- Iris
- The colored ring in the eye that opens and closes the pupil to control the amount of light entering.
- LASIK
- A refractive surgery that reshapes the cornea with a laser to reduce or remove the need for glasses.
- Lens (crystalline lens)
- The clear structure behind the pupil that fine-tunes the eye's focus.
- Macula
- The small central part of the retina responsible for sharp central vision.
- Myopia (nearsightedness)
- Far objects are harder to focus than near ones. Marked with "−" on a prescription.
- Ophthalmologist
- A medical doctor (MD or DO) who diagnoses and treats eye disease, prescribes medicine and performs surgery.
- Optic nerve
- The cable of nerve fibers that carries visual information from the retina to the brain.
- Optometrist
- A doctor of optometry (OD) who performs eye exams, prescribes glasses and contacts, and can treat many eye conditions.
- Presbyopia
- The age-related loss of near focus that starts in the 40s. Corrected with reading glasses, progressives or multifocal contacts.
- Pupil
- The round opening in the center of the iris that light passes through.
- Refraction
- The part of an eye exam that measures your glasses prescription.
- Retina
- The light-sensitive tissue at the back of the eye that sends visual signals to the brain.
- Retinal detachment
- The retina separates from the back of the eye. A medical emergency — treat sudden new floaters, flashes or a shadow in your vision as urgent.
- Sclera
- The tough white outer layer of the eye.
- Slit lamp
- The microscope your eye doctor uses to look at the front of your eye in detail.
- Strabismus
- Misalignment of the eyes — one turns in, out, up or down while the other looks ahead.
- Stye
- A painful red bump at the edge of the eyelid from a blocked oil gland.
- Tonometry
- The test that measures intraocular pressure. The "puff of air" or the gentle touch during an exam.
- Uveitis
- Inflammation of the middle layer of the eye. Causes pain, redness and light sensitivity. See a doctor promptly.
- Visual acuity
- Sharpness of vision, usually written as 20/20, 20/40 and so on. 20/20 means you see at 20 feet what a person with normal vision sees at 20 feet.
- Vitreous
- The clear gel that fills the main cavity of the eye between the lens and the retina.
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Sources
Definitions written from the National Eye Institute's Eye Conditions & Diseases pages and the American Academy of Ophthalmology's Eye Anatomy and Eye Diseases hubs. Every term links to a condition guide where one exists.