Vision Care Glossary: Common Eye Terms Explained

Visual acuity measures how clearly you can see fine detail at a set distance. The Snellen chart is the familiar poster with rows of letters that shrink from top to bottom. You read it from 20 feet away, and your score reflects the smallest line you can identify accurately.

A result of 20/20 means you see at 20 feet what a person with typical vision sees at 20 feet. A result of 20/40 means you need to be at 20 feet to see what others can see clearly from 40 feet away. The U.S. legal definition of blindness is 20/200 or worse in the better eye with the best possible correction in place, though many people who meet this definition still retain some usable vision.

Prescription abbreviations come from Latin and can look confusing at first. Once you know what each one means, the numbers on your prescription become much easier to follow.

• OD (oculus dexter) refers to your right eye
• OS (oculus sinister) refers to your left eye
• OU means both eyes together
• SPH (sphere) is the main lens power for nearsightedness or farsightedness
• CYL (cylinder) is the correction for astigmatism
• AXIS is the angle of the astigmatism correction, measured from 0 to 180 degrees
• ADD is the extra magnifying power for reading, used in bifocal and progressive lenses
• PD (pupillary distance) is the measurement in millimeters between your pupil centers, used to center lenses correctly in your frame

A diopter is the unit of measurement for lens power. The higher the number, the stronger the correction your eyes need. Prescriptions are written in 0.25-diopter steps.

IOP stands for intraocular pressure, the fluid pressure inside your eye. It is measured in millimeters of mercury (mmHg), and normal IOP generally falls between 10 and 21 mmHg. Elevated pressure is one of the main risk factors for glaucoma, which is why it is checked at every comprehensive eye exam.

Tonometry is the test used to measure IOP. The two most common methods involve either a small probe that briefly touches the eye surface after numbing drops are applied, or a quick puff of air directed at the eye. Both take only a few seconds and are painless.

Dilation means widening the pupil with special eye drops so your eye doctor can see the full structures at the back of the eye, including the retina, optic nerve, and blood vessels. Mydriasis is the medical term for this dilated state. After dilation, your near vision will be temporarily blurry and your eyes will be more sensitive to light for roughly 4 to 6 hours, so bringing sunglasses to your appointment is a good idea.

Small changes in prescription numbers from year to year are common and do not always mean your vision has gotten worse in a way you will notice day-to-day. What matters is whether the change is gradual and predictable or sudden and significant. A rapid change in prescription, especially in one eye, may warrant a closer look at underlying conditions such as cataracts, blood sugar changes, or other factors. Your eye doctor will let you know if a change requires further evaluation.

A vision screening checks basic visual acuity, typically using a simple chart, and is designed to flag obvious problems. It does not measure eye pressure, examine the retina, check for glaucoma, or evaluate ocular health. A comprehensive eye exam includes all of those things and is the only way to detect many serious eye conditions early, before symptoms develop. Screenings are useful for identifying people who need a full exam, but they are not a substitute for one.

Elevated IOP means more fluid pressure is building up inside the eye than is ideal. High pressure alone does not confirm glaucoma, but it is the most significant modifiable risk factor for developing it. Your eye doctor will look at several pieces of information together, including your optic nerve appearance, corneal thickness, and visual field results, before making any diagnosis or treatment recommendation. Some people have naturally higher pressure without nerve damage (a condition called ocular hypertension), while others develop glaucoma at pressures within the normal range.

Amblyopia is most effectively treated during early childhood when the visual system is still developing. Once that developmental window closes, the brain's ability to adapt becomes much more limited. Some adults with amblyopia may see modest improvement with certain vision therapy approaches, but results are generally more limited compared to treatment started in childhood. This is one of the key reasons why pediatric eye exams are recommended even for children who show no obvious signs of vision problems.

Occasional dryness and mild irritation can often be managed with over-the-counter lubricating drops, particularly preservative-free formulas used throughout the day. However, if your symptoms are frequent, affect your ability to read or use screens comfortably, cause redness or sensitivity to light, or do not improve with basic lubrication, it is worth discussing them with your eye doctor. Persistent dry eye can have multiple underlying causes, and a clinical evaluation can help identify whether prescription drops, in-office treatments, or lifestyle adjustments would be most effective for you.

A 20/40 result means you need to be at 20 feet to see clearly what a person with typical vision can see from 40 feet away. In practical terms, this level of acuity often makes tasks like reading road signs or seeing a whiteboard from across a room more difficult. Many states require at least 20/40 vision (with or without correction) for a standard driver's license. Your eye doctor will determine whether glasses, contacts, or another form of correction brings your vision to a clearer, more functional level.