Medicare comes with its own vocabulary, and terms like guaranteed issue, IRMAA, or the coverage gap can make us all gloss over with boredom. While we have tried very hard to keep Senior65.com jargon free, sometimes the Medicare acronyms are unavoidable. This Medicare glossary defines the 50 terms those new to Medicare run into most often. We have grouped by topic so you can quickly reference the area of your concern. Some cover Original Medicare’s four parts; others explain Medigap-specific rules like medical underwriting and the birthday rule. Bookmark this page and come back to it as new terms surface during enrollment or plan comparisons. We promise there will not be a test on this later.

What do all these Medicare terms mean?

The Parts of Medicare

  • Original Medicare: The traditional government-run program made up of Part A and Part B, letting you see any provider nationwide that accepts Medicare. When people say “original Medicare” they are talking about Medicare A and B.
  • Part A (Hospital Insurance): Covers inpatient hospital stays, skilled nursing facility care, hospice, and some home health care. Most people get it premium-free after paying Medicare taxes for at least 10 years.
  • Part B (Medical Insurance): Covers doctor visits, outpatient care, preventive services, and durable medical equipment. Everyone pays a monthly premium, which is $202.90 in 2026.
  • Part C (Medicare Advantage): A private-insurance alternative to Original Medicare that bundles Parts A and B (and usually Part D) into one plan with its own network and cost-sharing rules. Few call it Part C anymore because it just confuses matters.
  • Part D (Prescription Drug Coverage): Private drug plans that cover prescription medications, sold either as standalone plans alongside Original Medicare or bundled into a Medicare Advantage plan. Easy to remember because Drugs start with the letter D!

Enrollment Periods

  • Initial Enrollment Period (IEP): The seven-month window around your 65th birthday (three months before, your birthday month, and three months after) when you first sign up for Medicare.
  • General Enrollment Period (GEP): January 1 through March 31 each year, for people who missed their Initial Enrollment Period and don’t qualify for a Special Enrollment Period.
  • Annual Enrollment Period (AEP): October 15 through December 7 each year, when anyone can switch Medicare Advantage or Part D plans for the following year. Many people and organizations wrongly call this period Open Enrollment. AEP is not for switching Medigap Plans.
  • Medigap Open Enrollment Period: A one-time, six-month window starting the month you’re 65 or older and enrolled in Part B, when you can buy any Medigap policy sold in your state without medical underwriting. Sometimes this is also called your initial enrollment period. NOTE: in most states this is a one time occurrence so you don’t have a Open Enrollment Period each year.
  • Special Enrollment Period (SEP): A window outside the standard enrollment periods triggered by a qualifying event, such as losing employer coverage or moving out of a plan’s service area.

Costs and Payments

  • Premium: The amount you pay, usually monthly, to keep a Medicare plan or policy active, regardless of how many services you use.
  • Deductible: The amount you pay out of pocket before Medicare or your plan starts paying its share. The 2026 Part A deductible is $1,736 per benefit period; the Part B deductible is $283 per year. Most Medigap plans coverage the Part A deductible.
  • Coinsurance: Your percentage share of a covered service’s cost after you’ve met your deductible, commonly 20% under Part B.
  • Copayment: A flat dollar amount you pay for a covered service or prescription, common in Medicare Advantage, Part D and Medigap Plan N.
  • Out-of-Pocket Maximum: The most you’ll pay for covered services in a year before a plan covers 100% of costs. Original Medicare has no such cap on its own, unless you add additional coverage.
  • IRMAA (Income-Related Monthly Adjustment Amount): An extra amount added to your Part B and Part D premiums if your income is above a certain threshold, based on your tax return from two years earlier.
  • Part B Late Enrollment Penalty: A permanent premium increase for delaying Part B enrollment without qualifying employer coverage, generally 10% for each 12-month period you could have had Part B but didn’t. Source: Medicare.gov.
  • Part D Late Enrollment Penalty: A permanent premium increase for going 63 or more days without Part D or other creditable drug coverage after becoming eligible.

Medigap and Supplemental Coverage

  • Medigap (Medicare Supplement Insurance): A private policy that pays some or all of the out-of-pocket costs Original Medicare leaves behind, like deductibles and coinsurance. Also called Medicare Supplement, Medsupp, or Med Sup. This is what Senior65.com helps you enroll in. Give us a call if you want assistance!!!
  • Guaranteed Issue: A right that requires an insurance company to sell you a Medigap policy regardless of health history, typically during your Medigap Open Enrollment Period or after certain qualifying events.
  • Medical Underwriting: The health-history review a carrier uses to decide whether to approve a Medigap application, and at what price, outside guaranteed-issue windows. You can apply to switch Medigap plans anytime through underwriting; approval is possible even with some pre-existing conditions. The Medigap Underwriting Checker can show you how insurance providers would treat certain medical conditions before you apply.
  • Birthday Rule: A state-specific rule (California, Oregon, and a handful of others) letting current Medigap policyholders switch to an equal or lesser plan within a set window around their birthday, without underwriting. It only applies to Medigap-to-Medigap switches. See the Birthday Rule state guide for specifics.
  • Free Look Period: A 30-day window after buying a new Medigap policy to try it and cancel for a refund if it’s not a fit.
  • Trial Right: A protection for people who tried Medicare Advantage for the first time and switch back to Original Medicare within 12 months, letting them buy a Medigap policy with guaranteed issue rights. There are two versions of this Trial Right. Read more in our Medigap switching guide.
  • Standardized Plans (Plan Letters): Medigap policies labeled A through N, each offering the same federally standardized benefits no matter which insurance company sells it (Massachusetts, Minnesota, and Wisconsin standardize differently).
  • Community Rating: A Medigap pricing method that charges everyone in an area the same premium regardless of age, though premiums can still rise annually for other reasons. Many community-rated plans also offer disappearing discounts to younger members, so pricing can behave similarly to attained-age rating in practice.
  • Attained-Age Rating: A Medigap pricing method where premiums increase as you get older, on top of any annual rate increases that apply to everyone on the plan. Most plans in most states use this method.
  • Issue-Age Rating: A Medigap pricing method based on your age when you first bought the policy. Your age at purchase locks in, and premiums don’t increase further just because you’re aging. But like with all Medigap plans they can go up each year do to inflation. Bottom line: Medigap plans typically increase each year regardless of the pricing method so don’t overpay because you think one method is more appealing than another.
  • High-Deductible Plan G: A version of Medigap Plan G with a lower premium in exchange for paying a yearly deductible ($2,960 in 2026) before the plan starts paying its share.

💡 Tip: If you’re weighing whether to switch companies or plans, a Medigap quote takes less than 30 seconds and shows what you’d actually pay before you commit to anything.

Medicare Advantage and Plan Terms

  • HMO (Health Maintenance Organization): A plan type that generally requires you to use in-network providers and get referrals for specialists, often at a lower premium.
  • PPO (Preferred Provider Organization): A plan type that allows you to see out-of-network providers at a higher cost, typically without needing referrals.
  • Network: The group of doctors, hospitals, and other providers a plan has contracted with. Using providers outside the network can cost more, or may not be covered at all. See our Medicare Advantage vs. Medigap comparison for how this affects provider access.
  • Prior Authorization: A requirement that your plan approve certain services or medications before you receive them, common in Medicare Advantage plans.
  • Star Ratings: CMS’s 1-to-5 scale rating Medicare Advantage and Part D plans on quality and customer satisfaction, updated annually. Source: Medicare.gov.
  • Formulary: A plan’s list of covered prescription drugs, organized into cost tiers.

Prescription Drug Terms

  • Formulary Tiers: The cost levels within a formulary, generally running from lowest-cost generics (tier 1) to highest-cost specialty drugs (the top tier).
  • Coverage Gap (Donut Hole): A former Part D cost-sharing phase that no longer exists. Part D plans now have a single $2,100 annual out-of-pocket cap instead. Source: Medicare.gov.
  • Catastrophic Coverage: The Part D phase after you hit your yearly out-of-pocket cap, when you pay nothing more for covered drugs for the rest of the year.
  • Creditable Coverage: Drug or health coverage (such as from an employer) that Medicare considers at least as good as its own. Keeping it can help you avoid late enrollment penalties later. This is an official term so your HR department can confirm if your previous coverage was considered “Creditable” by Medicare.

Senior65 doesn’t typically sell Part D or Medicare Advantage. We can explain how it works in general and point you to Medicare.gov to compare and enroll in a standalone plan directly.

Assistance Programs

  • Extra Help (Low-Income Subsidy): A federal program that lowers Part D premiums, deductibles, and copayments for people with limited income and resources.
  • Medicaid: A joint federal-state program providing health coverage to people with limited income. Some Medicare beneficiaries qualify for both programs.
  • Dual-Eligible: A term for someone enrolled in both Medicare and Medicaid, who typically gets extra help with premiums and cost-sharing.
  • Medicare Savings Program (MSP): A state-run program that can pay Part B premiums, and sometimes other cost-sharing, for people with limited income who don’t qualify for full Medicaid.

Administrative Terms

  • CMS (Centers for Medicare & Medicaid Services): The federal agency that runs Medicare and Medicaid and sets the rules private insurers must follow to sell Medicare plans.
  • SSA (Social Security Administration): The agency that generally handles Medicare enrollment and premium billing, since Medicare and Social Security benefits are closely linked.
  • ABN (Advance Beneficiary Notice): A notice a provider gives you before a service if they believe Medicare might not cover it, so you can decide whether to proceed and accept the cost.
  • EOB (Explanation of Benefits): A statement from a Medicare Advantage or Part D plan showing what was billed, what the plan paid, and what you owe.
  • MSN (Medicare Summary Notice): A quarterly statement from Original Medicare listing the services billed on your behalf and what you may owe.
  • COBRA: A law letting you temporarily keep employer group health coverage after leaving a job. It generally doesn’t delay your need to enroll in Medicare on time, so the two need to be coordinated carefully.
  • Employer Group Health Plan: Health coverage through your or your spouse’s current, active employment, which can affect Medicare enrollment timing and may qualify you for a Special Enrollment Period later. Our Medicare switching guide covers how this timing works.

Whew….That is a lot of terms. We are hoping you skimmed and only read the areas that concerned you. Are you new to Medicare and want the fuller picture behind any of these terms? Our Medicare overview walks through how the parts fit together, and it’s a good next stop that isn’t so focused on vocab!

Get a Medigap Quote Enroll in Medigap

There’s never a fee to work with us, and no one can offer the same Medigap plan for less than Senior65.com. If a term in this glossary applies to your situation and you want to talk it through, contact our team at Senior65.com or call 800-930-7956.