Medicare Glossary

Plain-language definitions of common Medicare terms, updated with 2026 figures. Browse the list below or use your browser’s find feature.

Annual Enrollment Period (Fall Open Enrollment)

October 15 to December 7 each year, when you can join, switch or drop Medicare Advantage and Part D plans. Changes start January 1.

Benefit period

How Original Medicare measures your use of hospital and skilled nursing facility care. It starts the day you are admitted and ends after you have gone 60 days in a row without that care. The Part A deductible ($1,736 in 2026) applies to each benefit period.

Catastrophic coverage

The final Part D stage. In 2026, once your out-of-pocket costs for covered drugs reach $2,100, you pay $0 for covered Part D drugs for the rest of the year.

Coinsurance

Your share of the cost of a service, usually a percentage. For example, after the Part B deductible you typically pay 20% of the Medicare-approved amount.

Copayment

A fixed amount you pay for a covered service or drug, such as $20 for a doctor visit.

Creditable coverage

Drug or health coverage that is expected to pay at least as much as standard Medicare coverage. Having creditable drug coverage helps you avoid the Part D late enrollment penalty.

Deductible

The amount you pay for covered care or drugs before Medicare or your plan starts to pay. In 2026 the Part B deductible is $283 and no Part D plan deductible can be higher than $615.

Extra Help

A Medicare program that helps people with limited income and resources pay Part D premiums, deductibles and copays. Also called the Low-Income Subsidy (LIS).

Formulary

A plan’s list of covered prescription drugs, often grouped into tiers with different costs.

General Enrollment Period

January 1 to March 31 each year, for people who missed their Initial Enrollment Period for Part A or Part B.

Guaranteed issue rights

Situations in which a Medigap insurer must sell you certain policies and cannot charge more for health problems, such as when you lose other coverage.

HMO (Health Maintenance Organization)

A type of Medicare Advantage Plan that generally requires you to use doctors and hospitals in its network.

Initial Enrollment Period

Your first chance to sign up for Medicare: 7 months that begin 3 months before the month you turn 65.

IRMAA (Income-Related Monthly Adjustment Amount)

An extra amount people with higher incomes pay for Part B and Part D on top of the standard premium.

Late enrollment penalty

An amount added to your Part B or Part D premium if you did not sign up when first eligible and did not have other qualifying coverage.

Medical underwriting

When an insurer reviews your health history to decide whether to sell you a policy and at what price. Medigap insurers can use it in most states outside your Medigap Open Enrollment Period.

Medicare Advantage (Part C)

A Medicare-approved plan from a private company that provides Part A and Part B benefits and usually Part D drug coverage.

Medicare Part A

Hospital insurance. Covers inpatient hospital stays, skilled nursing facility care, hospice and some home health care.

Medicare Part B

Medical insurance. Covers doctor services, outpatient care, preventive services and durable medical equipment. The standard premium is $202.90 a month in 2026.

Medicare Part D

Prescription drug coverage offered by private plans approved by Medicare.

Medicare Supplement (Medigap)

Private insurance that helps pay costs Original Medicare does not, such as copayments, coinsurance and deductibles.

Medigap Open Enrollment Period

The 6 months starting the first month you have Part B and are 65 or older, when you can buy any Medigap policy sold in your state without health questions.

Network

The doctors, hospitals and pharmacies a plan contracts with to provide care.

Original Medicare

Part A and Part B, managed by the federal government. You can use any doctor or hospital in the U.S. that accepts Medicare.

Out-of-pocket limit

The most you will pay in a year for covered services. Medicare Advantage Plans each set a yearly limit; Part D has a $2,100 cap for covered drugs in 2026.

PPO (Preferred Provider Organization)

A type of Medicare Advantage Plan where you pay less in network and can go out of network for a higher cost.

Premium

The monthly amount you pay for coverage.

Prior authorization

Approval you or your provider must get from a plan before certain services or drugs are covered.

Special Enrollment Period

A time outside the usual enrollment periods when you can sign up or change coverage because of certain life events, such as losing job-based coverage or moving.

Special Needs Plan (SNP)

A Medicare Advantage Plan for people with specific diseases, people in institutions, or people with both Medicare and Medicaid.

Sources for 2026 figures: CMS 2026 Medicare Parts A & B Premiums and Deductibles; Medicare.gov: Medicare costs; Medicare.gov: Part D costs. For information on all of your options, contact Medicare.gov or 1-800-MEDICARE (TTY 1-877-486-2048).