Glossary Of Eye Care Terms - T-Z
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. Learn more about toric contact lenses.
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. Read more about UV light.
uvea Middle layer of the eye, below the limbus, and consisting of the iris, ciliary body and choroid. Read more about the uvea of the eye.
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).
Vision Council A major trade organization located in Alexandria, Virginia, that represents manufacturers and suppliers of the optical industry and is dedicated to educating the public about eye health and eyewear.
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).
vitrector Tiny, motorized cutting instrument used to remove the eye's gel-like vitreous
during a vitrectomy.
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. Eye 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 In optics, the surface connecting similar points on adjacent waves of light among an essentially infinite number of waves traveling together from a light source. The simplest form of a wavefront is called a plane wave, where all light rays are parallel to each other. Distortions of visual wavefronts caused by irregularities of the optical system of the eye are called higher-order aberrations (HOAs). These wavefront distortions can be measured at the plane of the pupil by instruments called aberrometers, which document HOAs in the form of a topographical map. These maps can then be used to correct HOAs with customized, wavefront-guided LASIK.
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 published in April 2019
Page updated in February 2021