Eyewear Glossary

Eyewear Terms & Definitions

24 terms defined — an authoritative reference for understanding the most important concepts in Eyewear.

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A

Add Power (ADD)

Add power (ADD) is the extra magnifying power added to the lower part of multifocal or reading lenses to help with close-up focus. It is a single plus number, often the same for both eyes, and is required to order progressive or bifocal glasses online.

Anti-Reflective (AR) Coating

Anti-reflective (AR) coating is a thin layer applied to lenses that cuts glare and reflections from screens, headlights, and overhead lights. It also makes lenses look clearer in photos. Most online retailers offer it as a paid add-on, and premium versions add scratch and smudge resistance.

Axis

Axis is a number from 1 to 180 that tells the lab which direction to place the cylinder correction for astigmatism. It only appears when your prescription lists a cylinder value. Entering the axis incorrectly when ordering online can leave your astigmatism uncorrected, so double-check it.

B

Base Curve (BC)

Base curve (BC) is the curvature of a contact lens, listed in millimeters on your contact prescription. It helps the lens match the shape of your eye for a comfortable, stable fit. Because it is set during a fitting, order the exact BC on your prescription rather than guessing.

Bifocal Lenses

Bifocal lenses split one lens into two powers: distance on top and a near-vision segment on the bottom, separated by a visible line. They are an alternative to progressives for people who want a wider reading area and do not mind the line. The reading power comes from your add value.

Blue-Light Filtering

Blue-light filtering is a lens treatment that reduces the blue-violet light reaching your eyes from screens and LED lighting. Many wearers choose it for comfort, but the American Academy of Ophthalmology does not recommend it to prevent eye damage, and evidence on easing eye strain is mixed.

C

Contacts vs Glasses Prescription

A contact lens prescription is not the same as a glasses prescription: it adds base curve, diameter, and a brand, and the powers can differ because contacts sit on the eye. You cannot order contacts using only your glasses prescription; you need a valid contact lens fitting from an eye doctor.

Cylinder (CYL)

Cylinder (CYL) is the amount of lens power added to correct astigmatism, which comes from an uneven cornea or lens shape. It is written as a plus or minus number in diopters. A blank CYL box usually means you have little or no astigmatism to correct.

D

Daily Disposable Contacts

Daily disposable contacts are worn once and thrown away at night, so there is no cleaning or storage. They are convenient and can suit people with allergies or sensitive eyes, but they cost more per year than reusable lenses. A year usually means several 90-pack boxes for each eye.

Diameter (DIA)

Diameter (DIA) is the width of a contact lens from edge to edge, in millimeters, shown on your contact lens prescription. Along with base curve, it determines how the lens sits on your eye. Use the exact diameter your eye doctor specified, since it is part of a proper contact lens fitting.

Dry Eye & MGD

Dry eye is when your eyes do not stay adequately lubricated, causing stinging, grittiness, or blurry vision; meibomian gland dysfunction (MGD) is a common cause. It can make contact lens wear uncomfortable. Warm compresses and lid hygiene may help, but see an eye doctor for persistent symptoms.

F

Frame / Contact Allowance

An allowance is a fixed dollar amount your vision plan puts toward frames or contacts each benefit period, such as a set amount toward a frame. You pay any difference above it, and unused amounts usually do not roll over. Allowances vary widely between plans, so check yours before shopping.

FSA / HSA

An FSA (Flexible Spending Account) or HSA (Health Savings Account) lets you pay for glasses, prescription sunglasses, contacts, and eye exams with pre-tax dollars. Many online retailers accept these cards directly or provide an itemized receipt you can submit for reimbursement.

H

High-Index Lenses

High-index lenses bend light more efficiently, so they can be thinner and lighter than standard plastic for the same prescription. They are worth considering for stronger prescriptions, where regular lenses look thick. Common indexes include 1.67 and 1.74, with higher numbers being the thinnest.

I

In-Network vs Out-of-Network

In-network means a retailer has a contract with your vision plan, so your benefit applies directly at checkout. Out-of-network means you pay up front and file a claim for partial reimbursement. Buying in-network usually stretches your allowance further, though selection may be more limited.

M

Monthly / Reusable Contacts

Monthly and biweekly contacts are worn on a set replacement schedule and cleaned nightly with solution, then stored in a case. They cost less per year than dailies but require disinfecting and timely replacement. Wearing them past their schedule raises the risk of eye infections.

P

Photochromic (Transition) Lenses

Photochromic lenses darken automatically in sunlight and clear indoors, so one pair works for everyday and bright light. They react to UV, which means most versions stay clear behind a windshield that blocks UV. Transitions is a common brand name, but several makers offer photochromic options.

Polarized Lenses

Polarized lenses contain a filter that blocks horizontal glare bouncing off water, snow, and roads, which makes them popular for driving and outdoor sports. They improve visual comfort in bright light but can make some LCD screens harder to read. Polarization is a sunglasses feature, not a prescription.

Prism

Prism is a lens correction that shifts the image slightly to help eyes that struggle to align, easing double vision or eye strain. It is listed with a power in prism diopters and a base direction. Prism prescriptions are specialized, so confirm the values with your eye doctor before ordering.

Progressive Lenses

Progressive lenses correct distance, intermediate, and near vision in one lens with no visible line, unlike bifocals. The power changes gradually from top to bottom. First-time wearers often need a short adjustment period, and a taller frame usually makes the near zone easier to use.

Pupillary Distance (PD)

Pupillary distance (PD) is the distance in millimeters between the centers of your pupils. Online opticians use it to center each lens over your eyes so your prescription lines up correctly. Most adult PDs fall between 54 and 74 mm, and you can measure it at home with a ruler and mirror.

S

Segment Height (Seg Height)

Segment height is the vertical distance, in millimeters, from the bottom of the lens to where the near or intermediate zone begins on progressive or bifocal lenses. It is measured on the frame you choose so the reading area sits correctly. Some online retailers help you estimate it.

Sphere (SPH)

Sphere (SPH) is the lens power that corrects nearsightedness or farsightedness, measured in diopters. A minus value such as -2.50 means you are nearsighted; a plus value such as +1.75 means you are farsighted. It appears once for each eye on your glasses prescription.

T

Toric Lenses

Toric contact lenses are shaped to correct astigmatism, with different powers in different meridians of the lens. Their prescription includes cylinder and axis values in addition to sphere. Toric lenses cost more than spherical ones and must be fitted by an eye doctor for stable vision.