Glossary of Eye Care Terms
reading glasses Also called readers. Glasses to help with close work, 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 When light rays don't properly refract from the cornea to the retina, it is a refractive error. This can take the form of myopia (nearsightedness), hyperopia (farsightedness) or astigmatism.
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.
replacement schedule How often you discard and replace your contact lenses: every day, week or two weeks (disposable); or every month, two months or calendar quarter (frequent replacement). It's important to differentiate between replacement schedule and wear schedule. Wear schedule is either daily wear (removed before sleeping) or extended wear (you may sleep with them in).
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 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 yourvision, blurred vision and vision loss. For more information, please see our retinal detachment article.
retinal pigment epithelium (RPE) A layer of pigmented cells found between the light-sensitive, inner back lining of the eye (retina) and the choroid, which contains blood vessels that supply nutrients and oxygen.
retinal tear A tear or split in the retina typically caused by a vitreous detachment. Symptoms include floaters and light flashes. For more information, please see our retinal detachment article.
retinitis Inflammation of the retina. Symptoms include blurred vision, metamorphopsia, floaters and vision loss.
retinitis pigmentosa Usually inherited condition characterized by progressive degeneration of the retina, resulting in night blindness and decreased peripheral vision. For more information, please see our retinitis pigmentosa article.
retinoschisis Condition in which the retina splits into layers, sometimes causing blurred vision. It is either inherited or acquired; the acquired form is caused by small cysts in the eye. You may also have floaters.
retrobulbar hemorrhage Bleeding in the eye socket behind the eye. If untreated, can cause increased intraocular pressure (IOP), protrusion of the eyeball and permanent vision loss.
rewetting drops Eye drops designed to re-moisten and lubricate contact lenses while they are being worn, to increase comfort.
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 now virtually obsolete.
rhegmatogenous Arising from a break or tear, describing a common type of retinal detachment.
rheopheresis The RHEO Procedure ("rheopheresis") is a method of blood filtration (apheresis) for treatment of dry age-related macular degeneration that removes large proteins and fatty components from the blood to improve circulation to macular cells at the back of the eye. For more information, please see our article on age-related macular degeneration.
rheumatoid arthritis Chronic, autoimmune, inflammatory disorder that mainly affects the joints. Symptoms include pain, stiffness, swelling and deformities. Rheumatoid arthritis may also cause eye burning, discharge and dryness. It can be associated with uveitis.
river blindness Onchocerciasis is caused by a parasitic worm, which is spread in the human bloodstream through bites from blackflies and buffalo gnats found in parts of Africa, South America, and Central America. The worm's offspring cause inflammation, bleeding, and other problems in the eye. Without a 15-year regimen of annual doses of Mectizan, blindness will result. (Information supplied by ORBIS International.)
RK (Radial Keratotomy) Surgical procedure where cuts are made in the cornea in a radial pattern, to flatten the cornea and correct myopia. However, RK now is virtually obsolete as a corrective eye procedure.
rod A photosensitive receptor in the retina that helps you to see in low light.
rosacea Skin condition typically involving the face that is characterized by flushing, red bumps and telangiectasia (dilated, visible capillaries); it is most common among fair-skinned women, who develop it in their 30s through 50s.
sarcoidosis Disease in which granulomas (nodules of inflamed tissue) form in the lymph nodes, lungs, skin and other areas. The cause is unknown. Symptoms include fatigue, weight loss, fever, coughing, uveitis, cranial nerve palsies and skin lesions. Some patients also develop dry eyes.
sclera The outer coat of the eyeball that forms the visible white of the eye and surrounds the optic nerve at the back of the eyeball.
scleritis Inflammation of the sclera. Autoimmune disorders are the most common cause. Symptoms include a red or pink eye, eye pain, light sensitivity, tearing and blurred vision.
scotoma Blind spot within the field of view.
seborrheic dermatitis Skin condition that causes scales, redness and itching; it commonly affects the scalp (dandruff), eyebrows, eyelids, nose, area behind the ears and sternum. It is associated with seborrheic blepharitis.
segment A part, as in the near-vision portion of a pair of bifocals.
silicone Type of flexible and comfortable plastic. Because it is commonly used in nose pads in 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 the entire lens, in contrast to a bifocal or multifocal lens that has 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 a dry mouth and dry eyes. Additional eye symptoms include burning, discharge, foreign body sensation, itching and light sensitivity.
skin cancer Skin cancer is commonly caused by exposure to the sun's UV rays. Types include basal-cell carcinoma, squamous cell carcinoma and malignant melanoma. In carcinoma, you are likely to have a red or pink bump that bleeds, crusts and scales. In melanoma, the pigmented areas can be raised or flat; they are often brown or black, but can be (or include) other colors as well, such as blue, red, pink or white. Some raised moles are skin-colored. The most common symptoms of a malignant mole follow an ABCD checklist:
- Asymmetry: one half does not match the other.
- Border: irregular or scalloped.
- Color: varies in shade or color.
- Diameter: larger than that of a pencil eraser.
Snellen chart Standard 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.
solution Product used to clean, disinfect and store contact lenses.
spatial frequency In optics and other fields, a measure of how often a repeating structure (such as a vertical line or bar) appears within a given unit of distance.
SPF (sun protection factor) Number representing the amount of sun something blocks. For example, if you normally begin to burn after half an hour in the sun, a sunscreen with an SPF of 2 should let you stay out twice as long (1 hour), SPF 4 should let you stay out four times as long, and so on.
spherical A contact lens design that is like a sphere and is fairly common; in contrast, toric lenses for astigmatism are football-shaped and are less common.
spots Small, cloudy specks in the eye that become noticeable when they fall in the line of sight. Read our spots and floaters article.
spring hinge Type of hinge on eyeglass frames that is more flexible than a regular hinge, making the frames more durable.
stereopsis Three-dimensional vision, enabling depth perception.
strabismus A misalignment of the eyes: the eye don't point at the same object together. Crossed eyes (esotropia) are one type of strabismus; "wall-eyes" (exotropia) are another. The exact cause is unknown, but appears to be a problem with the eye muscles. Strabismus can affect depth perception.
stroke An interruption in blood flow to the brain, resulting in tissue death. Blood clots and other obstructions can interrupt the blood flow, as can a ruptured artery. Symptoms include weakness or numbness, particularly on one side of the body, a change in consciousness or mental status, vision loss, double vision, and limited eye movement. You may also blink frequently.
stroma The cornea's middle layer; it consists of lamellae (collagen) and cells, and makes up most of the cornea.
sty 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.
superior limbic keratoconjunctivitis A condition characterized by episodes of recurrent inflammation of the superior cornea, the junction between the cornea and the sclera (the limbus), and the conjunctiva lining the superior sclera and inner surface of the upper eyelid.
surgery complication Complications from cataract surgery, LASIK or other eye surgeries can result in a variety of symptoms, including blurred vision, ptosis, foreign body sensation, halos around lights, light sensitivity, eye pain or discomfort, red or pink eyes, vision loss and an iris defect.
suspensory ligament Also called zonule of Zinn. Membrane of fibers (zonules) that holds the eye's lens in place.
Tay-Sachs disease Hereditary disorder resulting from a deficiency of the enzyme hexosaminidase. Symptoms include blindness, delayed development, seizures and paralysis. Tay-Sachs disease often results in an early death.
temple The "arm" of a pair of glasses, running from the ear to the lens area.
temporomandibular joint disorder Also called TMJ or TMD disorder. Problems with the jaw, jaw joint and surrounding muscles that control jaw movements such as chewing, which result in pain, swelling and difficulty with jaw movement. Other symptoms are toothaches, headaches and hearing problems. Causes include injury, teeth grinding/clenching, stress and osteoarthritis or rheumatoid arthritis.
thrombosis Blood clot. Such coagulation of the blood can occur anywhere in the circulatory system of the body, including the heart, arteries, veins and capillaries.
titanium A type of metal alloy that is very strong. Eyeglasses made of titanium are lightweight, durable and often hypoallergenic.
tonic pupil Dilated pupil that reacts sluggishly to light, due to damage to the ciliary ganglion from trauma, viral infections or other causes. The cause is sometimes unknown ("Adie's tonic pupil").
toric A lens design with two different optical powers at right angles to each other for the correction of astigmatism.
toxocariasis Infection caused by Toxocara worms, which are typically found in cat and dog intestines. The form found in the eyes, ocular larva migrans, can cause vision loss.
toxoplasmosis An infection caused by the Toxoplasma parasite, often from undercooked meat or contact with feces. It may occur in people with compromised immune systems. Symptoms are flu-like and can include swollen lymph nodes and muscle aches. Ocular toxoplasmosis causes inflammation of the eye's interior, leading to uveitis.
trachoma Chronic infection of the eyelid and cornea caused by a microorganism that is spread by contact with eye discharge from an infected sufferer. Flies can also transmit the bacteria. Over time, the eyelid becomes scarred and turns inward. The eyelashes begin to scrape the eyeball and cornea, which eventually causes visual impairment and blindness. Worldwide, 84 million people are affected by trachoma. Clean water and good hygiene can prevent trachoma, while antibiotics can treat it early on. Inward-turning eyelids can be corrected with simple surgery performed by a nurse. (Information supplied by ORBIS International.)
transient ischemic attack (TIA) Often called a "ministroke," a TIA is a short-lived blood clot-induced blockage of the blood supply to the brain. Vision may be blurred; other symptoms may include numbness on one side of the body, slurring of speech, dizziness and paralysis.
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 of the eye 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 (towards the eye).
trichotillomania Disorder characterized by compulsive hair or eyelash pulling. The exact cause is not known.
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.
20/20 vision Many eye care practitioners consider this the average visual acuity for human beings, but humans can see as well as 20/15 or even 20/10. People with 20/40 vision can see clearly at 20 feet what people with 20/20 vision can see clearly at 40 feet. In most of the United States, 20/40 is the lowest uncorrected acuity required for a driver's license.
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.
uvea Middle layer of the eye, below the limbus, and consisting of the iris, ciliary body and choroid.
uveitis Inflammation of the uvea. In most cases, the cause is unknown, 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. Read our uveitis article.
vascular birthmark A pink, red or purple mark (flat or slightly raised), typically on an infant's face or neck, caused by a malformation of blood vessels. Types of vascular birthmarks include capillary hemangiomas ("stork bites" or "angel's kisses") and port-wine stains.
vascular problem Problems with your body's vascular system (i.e., blood vessels, arteries and so on) can include hardening of the arteries, high blood pressure, a clot, an aneurysm, an embolus, etc. These problems can sometimes affect the eyes, resulting in such symptoms as blurred vision, a bulging eye, double vision, eye pain or discomfort, a red or pink eye, eyelid swelling and vision loss.
vector In gene therapy, an agent such as a modified virus used to carry and transport genetic material that is then transferred into recipient cells in an organism.
vergence disorder Vergence refers to the eyes' ability to turn either inward (convergence) or outward (divergence); convergence insufficiency is the most common vergence disorder. The disorders' exact causes 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 Part of the eye between the lens and the retina, containing 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. Read our spots and floaters article.
vitreous hemorrhage Bleeding that goes into 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.
wavefront Describes technology used to "map" how the eye processes images, enabling correction through surgery or lenses for obscure vision errors.
wear schedule How long you wear your contact lenses: either daily wear (you remove the lenses each night) or extended wear (you may sleep with them in). It's important to differentiate between wear schedule and replacement schedule that is, how often you discard and replace your lenses.
white dot syndrome One of a group of inflammatory conditions that are characterized by white dots in the retina and choroid. You may also hear these syndromes called by their specific names, such as acute posterior multifocal placoid pigment epitheliopathy, multiple evanescent white dot syndrome, birdshot chorioretinopathy or multifocal choroiditis and panuveitis. In some cases, the cause is unknown; in others, it's believed to be an autoimmune disorder. Symptoms can include blurred vision, loss of color vision, floaters, light sensitivity, metamorphopsia and vision loss.
wraparound Also called "wrap" for short. Type of eyeglass or sunglass frame that curves around the head, from the front to the side. Wraparound sunglasses offer extra sun and wind protection at the sides. Most cannot accept prescription lenses, because the curvature causes optical distortion. However, some of the newer styles have been engineered to overcome this problem.
xanthelasma A yellow, fatty spot or bump on the inner corner of either the upper eyelid, the lower one or both eyelids, 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 investigated for a possible association with promoting healthy vision. [Read more about zeaxanthin.]
zyl Zylonite, or cellulose acetate, is a lightweight plastic often used in eyeglass frames. It often appears in laminated form, with layers in different colors, but it can also be made in mottled patterns to imitate natural tortoise shell or animal skins. More about eyeglass frame materials.
[Page updated August 2012]
For more Eye and Vision Information articles, please visit this section's home page or use the search box below.