Medicare Glossary
Plain-language definitions of the Medicare terms you'll encounter most. Have a question about how one applies to you? Ask a licensed advisor.
Last updated Jul 11, 2026
- Annual Enrollment Period (AEP)
- The window from October 15 to December 7 each year when anyone with Medicare can join, switch, or drop a Medicare Advantage or Part D prescription drug plan. Changes take effect January 1.
- Initial Enrollment Period (IEP)
- The 7-month window to first sign up for Medicare — it begins 3 months before the month you turn 65, includes your birthday month, and ends 3 months after.
- Special Enrollment Period (SEP)
- A time outside normal enrollment windows when you can change coverage due to a qualifying life event, such as losing employer coverage or moving out of your plan's service area.
- General Enrollment Period (GEP)
- January 1 to March 31 each year, when people who missed their Initial Enrollment Period can enroll in Part A and/or Part B. Late penalties may apply.
- Medicare Part A
- Hospital insurance covering inpatient hospital stays, skilled nursing facility care, hospice, and some home health care. Most people pay no premium for Part A.
- Medicare Part B
- Medical insurance covering doctor visits, outpatient care, preventive services, and durable medical equipment. Part B has a standard monthly premium.
- Medicare Part C (Medicare Advantage)
- An 'all-in-one' alternative to Original Medicare offered by private insurers. Plans bundle Parts A and B, usually include Part D, and often add dental, vision, and hearing benefits, typically using provider networks.
- Medicare Part D
- Prescription drug coverage offered through private plans. Each plan has a formulary (list of covered drugs). Going without creditable drug coverage can trigger a late enrollment penalty.
- Medicare Supplement (Medigap)
- Private insurance that works alongside Original Medicare to help pay out-of-pocket costs like coinsurance and deductibles. Medigap plans let you see any provider nationwide who accepts Medicare.
- Original Medicare
- The federal program made up of Part A and Part B. You can add a separate Part D plan and, if you wish, a Medigap policy.
- Deductible
- The amount you pay out of pocket for covered services before your plan begins to pay.
- Copayment (Copay)
- A fixed dollar amount you pay for a covered service, such as $20 for a doctor visit.
- Coinsurance
- Your share of the cost of a covered service, calculated as a percentage (for example, 20%).
- Maximum Out-of-Pocket (MOOP)
- The most you will pay for covered medical services in a year under a Medicare Advantage plan. After you reach it, the plan pays 100% of covered services.
- Formulary
- The list of prescription drugs a Part D or Medicare Advantage plan covers, organized into cost tiers.
- Creditable Coverage
- Prescription drug (or other) coverage that is at least as good as Medicare's standard benefit. Keeping creditable coverage lets you delay enrollment without a late penalty.
- Late Enrollment Penalty (LEP)
- A permanent amount added to your Part B or Part D premium if you go without creditable coverage after you were first eligible. Medicare.gov — Part D late enrollment penalty.
- Guaranteed Issue Rights
- Situations in which insurers must sell you a Medigap policy without medical underwriting, such as during your Medigap Open Enrollment Period.
- Medical Underwriting
- The process insurers may use to review your health when you apply for a Medigap policy outside a guaranteed-issue window, which can affect approval or price.
- Provider Network
- The doctors, hospitals, and pharmacies a Medicare Advantage plan contracts with. Using in-network providers usually costs less.
- Star Ratings
- Medicare's 1-to-5 quality rating for Medicare Advantage and Part D plans, based on care quality, member experience, and customer service.
- SHIP (State Health Insurance Assistance Program)
- A free, unbiased counseling service that helps people with Medicare understand their options, available in every state. SHIP Help — find your local counselor.