Vision Care Glossary
Refractive Errors and Vision Conditions
Myopia (nearsightedness) means distant objects look blurry while close ones stay clear. Your eye is slightly too long from front to back, so light focuses in front of the retina instead of on it. Glasses or contacts with a negative-power lens push the focus point back onto the retina.
Hyperopia (farsightedness) is the opposite: close objects blur because your eye is slightly too short. A positive-power lens corrects this by pulling the focus point forward. Young patients can compensate for mild hyperopia by using their focusing muscles, which is why it may go undetected until adulthood.
Astigmatism develops when your cornea (the clear front surface of your eye) or lens has an uneven curve. This causes blurry or distorted vision at all distances. Your doctor corrects astigmatism with a cylinder and axis value added to your prescription.
Presbyopia is the gradual loss of near-focusing ability that begins around age 40. The lens inside your eye stiffens with age, making it harder to shift focus from distance to near objects. Reading menus, phone screens, and books becomes difficult without reading glasses or bifocals.
Presbyopia affects everyone, regardless of whether you had perfect vision before. Your doctor corrects it by adding an 'add power' to your prescription for close work. Progressive lenses blend distance and near correction so you can see at all ranges without switching glasses.
Accommodative dysfunction is a related term used when focusing problems appear in younger patients due to eye muscle fatigue. Your doctor distinguishes between presbyopia and accommodation issues through specific testing during your exam.
Amblyopia (lazy eye) is reduced vision in one eye that develops when the brain favors the other eye during childhood. Treatment works best when started early, before age 7. Your doctor may prescribe patching the stronger eye, special glasses, or eye drops to strengthen the weaker eye.
Strabismus (crossed eyes or wall eyes) is a misalignment where the eyes point in different directions. Some children are born with strabismus, while others develop it later. Your doctor evaluates eye muscle balance and may recommend glasses, vision therapy, or surgery depending on the type and severity.
Diplopia (double vision) can result from strabismus or from nerve and muscle problems that prevent your eyes from working together. Sudden-onset double vision in an adult requires prompt evaluation, as it can signal a neurological issue.
Eye Exam Terms and Measurements
Visual acuity measures how well you see detail at a specific distance. The Snellen chart is the familiar poster with rows of letters that get smaller from top to bottom. You read the chart from 20 feet away (or through mirrors that simulate 20 feet), and your score reflects the smallest line you can read.
A result of 20/20 means you see at 20 feet what someone with normal vision sees at 20 feet. A result of 20/40 means you need to stand at 20 feet to see what others see at 40 feet. The U.S. legal definition of blindness is 20/200 or worse in the better eye with best correction.
A diopter is the unit that measures lens power in your prescription. The higher the number, the stronger the correction your eyes need. Prescriptions are written in 0.25-diopter increments.
OD stands for the Latin 'oculus dexter' and refers to your right eye. OS stands for 'oculus sinister' and means your left eye. OU means both eyes together. Your prescription lists values separately for OD and OS because each eye needs a different correction.
SPH (sphere) is the main lens power for nearsightedness or farsightedness. CYL (cylinder) is the correction for astigmatism. AXIS is the angle of your astigmatism correction, ranging from 0 to 180 degrees. ADD is the extra magnifying power for reading, used in bifocal and progressive lenses.
PD (pupillary distance) is the distance in millimeters between the centers of your pupils. According to clinical references, the average PD is approximately 62 mm for women and 64 mm for men. Accurate PD ensures your lenses are centered in the frame.
IOP (intraocular pressure) is the fluid pressure inside your eye, measured in millimeters of mercury (mmHg). Normal IOP falls between 10 and 21 mmHg. Elevated pressure is a major risk factor for glaucoma and is checked at every comprehensive exam.
Tonometry is the test your doctor uses to measure IOP. The most common methods use a small probe that touches the eye surface after numbing drops, or a puff of air directed at the eye. Both take only seconds.
Dilation refers to widening your pupils with eye drops so your doctor can see the structures at the back of your eye. Mydriasis is the medical term for the dilated state of the pupil. Effects include blurry near vision and light sensitivity for 4 to 6 hours.
Eye Diseases and Treatments
Glaucoma is a group of diseases that damage the optic nerve, the bundle of nerve fibers connecting your eye to your brain. Most types of glaucoma develop without symptoms until vision loss occurs. Your doctor monitors eye pressure, optic nerve appearance, and visual field testing to detect and track glaucoma.
Age-related macular degeneration (AMD) damages the macula, the small central area of the retina responsible for sharp straight-ahead vision. Dry AMD progresses slowly as the macula thins. Wet AMD causes faster damage when abnormal blood vessels leak fluid under the retina.
Cataracts are a clouding of the natural lens inside your eye. Most cataracts develop gradually with age and cause blurry vision, glare, and faded colors. Cataract surgery replaces the cloudy lens with a clear artificial one.
LASIK (laser-assisted in situ keratomileusis) reshapes the cornea to correct myopia, hyperopia, and astigmatism. Your surgeon creates a thin flap in the cornea, reshapes the tissue underneath with a laser, and replaces the flap. Most patients achieve improved vision within a day.
Phacoemulsification is the standard technique for cataract surgery. Your surgeon uses an ultrasound probe to break up the cloudy lens and suction out the pieces. A clear artificial lens (intraocular lens or IOL) replaces the natural one.
Anti-VEGF injections treat wet AMD, diabetic macular edema, and other conditions caused by abnormal blood vessel growth. VEGF stands for vascular endothelial growth factor, a protein that triggers new blood vessel formation. The injection blocks this protein and reduces leaking and swelling in the retina.
Your eye doctor may prescribe drops to lower eye pressure (for glaucoma), reduce inflammation (steroid drops), fight infection (antibiotic drops), or manage allergies (antihistamine drops). Each type targets a specific problem and requires different usage instructions.
Cycloplegic drops temporarily paralyze the focusing muscle inside the eye, used during exams to get accurate prescription readings in children. Mydriatic drops dilate the pupil for retinal examination. Your doctor chooses the appropriate drop based on your age, condition, and the purpose of the exam.
Artificial tears are over-the-counter drops that lubricate the eye surface and relieve dry eye symptoms. Your doctor may recommend preservative-free versions for frequent use, as preservatives can irritate sensitive eyes over time.
Vision Glossary Questions
OD is your right eye and OS is your left eye. These abbreviations come from Latin terms. OU means both eyes. Your prescription and exam results list findings for each eye separately under these labels.
Nearsighted (myopic) patients see close objects well but distant ones appear blurry. Farsighted (hyperopic) patients struggle more with close-up tasks. Both conditions are corrected with lenses: negative power for nearsightedness and positive power for farsightedness.
IOP stands for intraocular pressure, the fluid pressure inside your eye. Elevated pressure can damage the optic nerve and lead to glaucoma. Your doctor measures IOP at every comprehensive exam to catch high pressure before it causes harm.
Legal blindness in the United States is defined as 20/200 or worse visual acuity in the better eye with best correction, or a visual field of 20 degrees or less. This is a legal definition used for benefits eligibility, not a medical diagnosis. Many people who meet this definition still have some usable vision.
Refraction is the bending of light as it passes through your eye. Your doctor measures how your eye bends light to determine the lens power that gives you the clearest vision. The refraction test, done with a phoropter, produces the numbers on your eyeglass prescription.
Dry AMD involves gradual thinning of the macula and slow vision loss. Wet AMD involves abnormal blood vessels leaking fluid under the retina, causing faster and more severe damage. Your doctor monitors for conversion from dry to wet AMD at regular exams, since wet AMD requires prompt treatment.
Talk to Your Eye Doctor
Understanding eye care terms helps you follow your exam results and make informed decisions about your vision. Bring this glossary to your next appointment and ask your doctor about any terms you want explained further.