Eye Care Glossary

Eye care has its own vocabulary. Whether you're reading your prescription for the first time, following up on a condition, or simply curious, this glossary explains the terms our optometrists use most often — in plain English.

Amblyopia (Lazy Eye): A condition in which one eye fails to develop normal visual acuity during childhood, typically because the brain begins favoring the stronger eye and suppressing signals from the weaker one. Causes include strabismus (eye turn), a large difference in prescription between eyes, or a cataract blocking vision in one eye. Most effectively treated before age 8 with glasses, patching, or vision therapy.
Astigmatism: A common refractive error caused by an irregularly shaped cornea or lens — more like a football than a round ball. Instead of focusing light to a single point, astigmatism causes light to focus at two different points, resulting in blurred or distorted vision at all distances. Corrected with glasses, toric contact lenses, or refractive surgery.
Axis: A value on your eyeglass or contact lens prescription (a number from 1 to 180) that indicates the orientation of the astigmatism correction. Axis always appears alongside the cylinder (Cyl) value. Even a small error in axis can cause distorted vision and headaches, which is why precise measurement matters.

Base Curve: The measurement of curvature on the back surface of a contact lens, expressed in millimeters. The base curve must closely match the natural curvature of your cornea for the lens to fit properly, provide accurate vision, and remain comfortable throughout the day. This is why contact lens prescriptions require a separate fitting.
Blue Light: High-energy visible (HEV) light with wavelengths between approximately 380 and 500 nanometers, emitted by digital screens, LED lighting, and the sun. Blue light at night suppresses melatonin production and can disrupt sleep. Blue light filtering lens coatings reduce screen glare and may improve comfort during extended digital device use.

Cataract: A clouding of the eye's natural crystalline lens, typically caused by the gradual oxidation of lens proteins with age. UV exposure, certain medications, and metabolic conditions can accelerate the process. Cataracts cause progressive blurring, glare, halos, and faded color perception. The only treatment is surgical removal and replacement with a clear intraocular lens (IOL) — one of the most commonly performed surgeries in the world.
Cornea: The clear, dome-shaped front surface of the eye. The cornea is responsible for approximately 70% of the eye's total focusing power and is the first structure light passes through when entering the eye. It contains no blood vessels and receives oxygen directly from the air. Many refractive surgeries (LASIK, PRK) permanently reshape the cornea to reduce or eliminate the need for glasses.
Cylinder (Cyl): The component of an eyeglass or contact lens prescription that corrects astigmatism. It indicates the degree of lens power needed to compensate for an irregularly shaped cornea or lens. Cylinder always appears with an axis value that defines the orientation of the correction. A zero cylinder value means no astigmatism correction is needed.

Diabetic Retinopathy: A complication of diabetes that damages the tiny blood vessels supplying the retina. Elevated blood sugar weakens these vessels, causing them to leak, swell, or stimulate abnormal growth. Early stages produce no symptoms, making annual dilated eye exams essential for all patients with Type 1 or Type 2 diabetes. Left untreated, it is a leading cause of vision loss in working-age adults.
Diopter (D): The unit of measurement used to express the refractive (focusing) power of a lens. Negative diopter values correct myopia (nearsightedness); positive values correct hyperopia (farsightedness). For example, a prescription of −3.00 D corrects moderate nearsightedness, while +2.25 D corrects farsightedness. The further from zero, the stronger the lens required.
Dilation (Mydriasis): The use of medicated eye drops to temporarily widen (dilate) the pupils. Dilation allows the optometrist a much wider, clearer view of the retina, optic nerve, macula, and peripheral retinal structures. A dilated exam is the most thorough way to evaluate the health of the back of the eye and is particularly important for patients with diabetes, glaucoma risk, or a family history of retinal disease.
Dry Eye Syndrome: A chronic condition in which the eyes fail to produce sufficient tears, or when the tears produced evaporate too quickly due to a deficiency in the oily outer layer of the tear film. Symptoms include grittiness, burning, redness, and paradoxically — excessive watering. Arizona's dry, high-UV climate makes it one of the most commonly treated conditions at our North Scottsdale clinic. Highly treatable with proper diagnosis.

Glaucoma: A group of eye diseases that damage the optic nerve, most often caused by elevated intraocular pressure. Glaucoma typically produces no early symptoms — vision loss begins peripherally and progresses silently until substantial damage has occurred. It is sometimes called "the silent thief of sight." Early detection through regular IOP measurement and optic nerve evaluation is critical, as vision loss from glaucoma is irreversible.

Hyperopia (Farsightedness): A refractive error in which the eye is slightly too short or the cornea too flat, causing light to focus behind the retina rather than precisely on it. Mild hyperopia can be temporarily compensated by the focusing muscles, but leads to eye strain, fatigue, and headaches — particularly during prolonged reading or close-up tasks. Corrected with plus-powered (convex) lenses or contact lenses.

Intraocular Pressure (IOP): The fluid pressure within the eye, maintained by the continuous production and drainage of aqueous humor — the clear fluid that fills the front chamber. Normal IOP is generally between 10 and 21 mmHg. Elevated IOP is the primary risk factor for glaucoma, though glaucoma can also develop at normal pressure levels. IOP is measured at every comprehensive eye exam using tonometry.

Macular Degeneration (AMD): An age-related condition in which the macula — the central zone of the retina responsible for sharp central vision — gradually deteriorates. The dry form involves slow macular thinning; the wet form involves abnormal blood vessel growth and leakage that can cause rapid central vision loss. AMD is the leading cause of vision loss in adults over 50 in the United States. Annual retinal imaging is the key to early detection.
Myopia (Nearsightedness): The most common refractive error worldwide, in which the eye is slightly too long or the cornea too steeply curved, causing distant objects to appear blurry while near objects remain clear. Myopia typically develops in childhood and stabilizes in the mid-20s. It is corrected with minus-powered (concave) lenses, contact lenses, or refractive surgery such as LASIK.

OCT (Optical Coherence Tomography): An advanced, non-invasive imaging technology that produces high-resolution cross-sectional images of the retina and optic nerve — similar in concept to an ultrasound, but using light waves instead of sound. OCT detects subtle structural changes associated with macular degeneration, glaucoma, and diabetic retinopathy, often before any symptoms are apparent. It allows year-over-year comparison of retinal thickness and optic nerve structure.
OD / OS: Latin abbreviations used in eye care prescriptions and records. OD (oculus dexter) refers to the right eye; OS (oculus sinister) refers to the left eye; OU (oculi uterque) refers to both eyes together. Some modern prescriptions use RE (right eye) and LE (left eye) instead, though OD/OS remain the most common abbreviations in clinical settings.

Photochromic Lenses: Prescription lenses that automatically darken in UV light (outdoors) and return to clear indoors — providing both vision correction and sun protection in a single lens. Common brand names include Transitions®. Note: most photochromic lenses do not darken inside vehicles, because car windshields block the UV radiation that triggers the darkening reaction.
Polarized Lenses: Lenses with a special filter that blocks horizontal light waves causing glare from reflective surfaces such as roads, water, and dashboards. Polarization dramatically reduces eye strain while driving and improves visual clarity outdoors — especially in Arizona's intense sunlight. Importantly, polarization is a glare-reduction feature and does not automatically mean UV protection; look for both UV400 and polarized on quality sunglasses.
Presbyopia: The age-related loss of near-focusing ability caused by gradual stiffening of the eye's crystalline lens. Presbyopia affects virtually everyone, typically becoming noticeable in the early-to-mid 40s when reading and close-up tasks become difficult without reading glasses. It is not a disease — it is a normal biological change. Corrected with reading glasses, bifocals, progressive lenses, or multifocal contact lenses.
Progressive Lenses (No-Line Bifocals): Multifocal lenses that provide a seamless gradient of correction from distance vision at the top of the lens, through intermediate (computer) distance in the middle, to near vision at the bottom — without the visible line of traditional bifocals. Progressive lenses require an adjustment period as the brain learns to use different zones. Precise pupillary distance measurement and frame fitting are critical for comfort.
Pupillary Distance (PD): The distance in millimeters between the centers of the two pupils. PD is not part of the clinical prescription but is required to correctly center the optical lenses in your frames. An incorrect PD causes the lenses to be optically off-center, producing visual distortion and eye strain — particularly noticeable in higher-prescription lenses and progressive lenses.

Refraction: The clinical test used to determine your precise eyeglass or contact lens prescription. Using a phoropter — the large, mask-like instrument with interchangeable lenses — your optometrist presents a series of lens combinations until you identify the one producing your clearest, most comfortable vision. This is the source of the familiar "which is clearer — one or two?" sequence during an eye exam.
Retina: The light-sensitive tissue lining the inner back surface of the eye. The retina functions like the sensor in a digital camera — it converts incoming light into electrical signals that travel via the optic nerve to the brain, which interprets them as images. The central retina (macula) handles sharp central vision and color; the surrounding peripheral retina handles side vision and low-light detection.

Sphere (Sph): The primary component of an eyeglass prescription that corrects nearsightedness or farsightedness. A negative sphere value (e.g., −3.00) corrects myopia; a positive value (e.g., +2.25) corrects hyperopia. A prescription with only a sphere value and no cylinder means the eye has no significant astigmatism. The larger the absolute number, the more powerful the lens correction required.
Strabismus (Crossed Eyes): A condition in which the two eyes do not point in the same direction simultaneously. One eye may turn inward (esotropia), outward (exotropia), upward, or downward. Strabismus can cause double vision or amblyopia (lazy eye) as the brain suppresses the image from the misaligned eye. Most effectively treated in early childhood with corrective glasses, patching, vision therapy, or surgical muscle correction.

Tonometry: The clinical measurement of intraocular pressure (IOP), performed as part of every comprehensive eye exam to screen for glaucoma. The most common method is non-contact tonometry — the brief "puff of air" test — which measures the eye's resistance to a gentle air pulse. Applanation tonometry uses a small probe at the eye surface after numbing drops are applied and provides a more precise reading for patients with elevated pressure.

UV400: A lens protection standard indicating that the lenses block all ultraviolet radiation with wavelengths up to 400 nanometers — covering both UV-A and UV-B rays completely. UV400 is the minimum standard recommended by eye care professionals for sunglasses. A dark tint does not imply UV protection; always look for the UV400 label. Long-term UV exposure is a major risk factor for cataracts and macular degeneration.

Visual Acuity: A measurement of the clarity and sharpness of vision at a specific distance, typically recorded using the Snellen notation (e.g., 20/20, 20/40). "20/20" means you can see at 20 feet what a person with normal vision sees at 20 feet. "20/40" means you must be 20 feet away to see what a normal-sighted person can see at 40 feet. Visual acuity is only one measure of visual function — it does not reflect eye health, peripheral vision, depth perception, or color vision.
Vitreous: The clear, gel-like substance that fills the interior of the eye between the lens and the retina, giving the eye its shape and volume. As we age, the vitreous gradually liquefies and may pull away from the retina — a process called posterior vitreous detachment (PVD). PVD often causes the sudden appearance of new floaters and flashes of light. While PVD itself is usually harmless, it warrants a prompt dilated exam to confirm the retina remains intact.

Let Our Optometrists Explain It In Person