Updated August 6, 2026. Quick answer: Medicare’s vocabulary hides its most expensive rules inside ordinary-sounding words — “benefit period” and “observation status” both cost real money. 40 terms, each with its own link anchor so a single definition can be cited directly.
The terms
Original Medicare. Part A plus Part B, run by the federal government, usable at any provider that accepts Medicare. It is one of the two roads; Medicare Advantage is the other, and the choice between them is the biggest one you make. Full page.
Part A (hospital insurance). Inpatient hospital, skilled nursing after a qualifying stay, hospice and some home health. Most people pay no premium for it because they paid for it through payroll tax.
Part B (medical insurance). Doctors, outpatient care, tests and durable medical equipment. It carries a monthly premium for everyone, and a surcharge for higher earners. Full page.
Part C (Medicare Advantage). A private plan that replaces Original Medicare and usually bundles drug coverage. It trades an out-of-pocket cap and extra benefits for a network and prior authorisation. Full page.
Part D (prescription drug coverage). Private drug coverage, either standalone or inside an Advantage plan. Which drugs are covered and at what price is decided by the plan’s formulary, not by Medicare. Full page.
Medigap (Medicare Supplement). Private insurance that pays the gaps in Original Medicare. Plans are standardised by letter, so the same letter buys the same coverage from any insurer — only the price and the service differ. Full page.
Deductible. What you pay before coverage starts paying. Part A’s resets per benefit period, not per year — which is a genuinely different thing and can happen more than once in a calendar year.
Coinsurance. Your percentage share of a cost after the deductible. Part B’s runs indefinitely under Original Medicare with no cap, which is the main reason Medigap exists.
Copayment. A flat per-visit or per-prescription charge, typical of Advantage and Part D plans. Easier to predict than coinsurance, which is part of the appeal.
Benefit period. The Part A clock. It starts on admission and ends after 60 consecutive days out of a hospital or skilled nursing facility. A new benefit period means a new deductible, so two separate admissions in one year cost more than people budget for.
Lifetime reserve days. A one-time bank of extra inpatient days, usable once across your whole life. Once spent, they are gone.
Skilled nursing facility (SNF) care. Short-term rehabilitative care after a qualifying inpatient hospital stay. It is not long-term care, and Medicare does not pay for long-term care — the most consequential misunderstanding in this entire vocabulary.
Observation status. Being in a hospital bed while formally classed as an outpatient. It is billed under Part B, and it can fail to count toward the inpatient stay that Part A requires before it will pay for skilled nursing. Ask about status, in words, while you are still there.
Assignment. A provider accepting Medicare’s approved amount as full payment. Most do. Those who do not may bill you more.
Excess charges. The extra a non-participating provider may bill above Medicare’s approved amount. Some Medigap letters cover them and some do not, and some states forbid them entirely. Full page.
Network. The providers an Advantage plan will pay for. HMO plans generally pay for nothing outside it except emergencies; PPO plans pay less. Networks change annually, including mid-treatment. Full page.
Prior authorisation. The plan’s requirement to approve care before it is given. Original Medicare uses it rarely; Advantage plans use it routinely, and it is the most common source of denial. Full page.
Formulary. The plan’s list of covered drugs. If your drug is not on it, the plan does not pay, and formularies change every year — which is why the same plan can be right one year and wrong the next. Full page.
Tier. The formulary band a drug sits in, which decides your share. The same drug can be a different tier in two plans with identical premiums.
Out-of-pocket cap. A ceiling on what you pay in a year. Part D has one, and Advantage plans have one for medical costs. Original Medicare on its own has none — that asymmetry is the whole argument for Medigap. Full page.
IRMAA (income-related monthly adjustment amount). The income surcharge on Part B and Part D premiums. It is a cliff, not a gradient: one dollar over a threshold moves you a whole bracket. Full page.
IRMAA two-year lookback. IRMAA is set from your tax return two years earlier, so a one-off event follows you with a two-year delay. Planning for it happens two years before you feel it. Full page.
Life-changing event. The specific list of circumstances that let you ask for IRMAA to be recalculated on current income — retirement, widowhood, divorce and a few others. A Roth conversion is not on it. Selling a business is not on it. Full page.
New initial determination. The correct route for asking Social Security to redo an IRMAA decision after a qualifying life-changing event. It is not an appeal, and asking for the wrong one is why people are told no. Full page.
Initial enrolment period (IEP). The seven-month window around your 65th birthday. Missing it is what creates a lifetime penalty, and it does not forgive people who simply did not know. Full page.
General enrolment period (GEP). The catch-up window for people who missed their IEP and have no special right. Coverage starts later and a penalty usually attaches.
Special enrolment period (SEP). A right to enrol outside the normal windows because of a specific event — most commonly losing employer coverage. It is what lets people work past 65 without penalty, provided the employer coverage actually qualifies. Full page.
Annual enrolment period (open enrolment). The autumn window for changing Part D and Advantage plans for the following year. It is the only routine chance most people get, and plans change underneath you every year whether you act or not. Full page.
Medicare Advantage open enrolment. A separate early-year window for people already in an Advantage plan to switch or return to Original Medicare. It is not the same window as the autumn one and it does not do the same things. Full page.
Late enrolment penalty. A permanent premium increase for enrolling late in Part B or Part D. Permanent means for as long as you have the coverage, not for a year. Full page.
Creditable coverage. Drug coverage at least as good as Part D. Holding it is what protects you from the Part D penalty while you delay — and employers are required to tell you each year whether yours qualifies. Keep the letter. Full page.
Guaranteed issue right. A situation in which a Medigap insurer must sell to you regardless of health. Outside those situations, and outside your one-time open window, they generally need not. Full page.
Medical underwriting. The health questions a Medigap insurer may ask outside a guaranteed-issue situation. This is the trapdoor in the Advantage-versus-Medigap decision: switching back to Medigap later may not be available at any price. Full page.
Birthday rule. A state rule letting you change Medigap plans around your birthday without underwriting. Only some states have one, and their terms differ. Full page.
Trial right. A limited right to leave a first Medicare Advantage plan and return to Original Medicare with Medigap, without underwriting. It is time-limited and it is one of the few second chances in this system. Full page.
Extra Help (Low-Income Subsidy). Help with Part D costs for people under income and asset limits. It is applied for through Social Security, and take-up is far below eligibility.
Medicare Savings Programs. State programmes that pay Medicare premiums, and sometimes cost-sharing, for people under income limits. Separate from Extra Help, though qualifying for one often opens the other.
Durable medical equipment (DME). Equipment for use at home — walkers, oxygen, hospital beds. Covered under Part B, usually with coinsurance, and usually only from a supplier that accepts assignment.
Appeal. The formal process for challenging a coverage denial. It runs in levels, each with its own deadline, and the deadlines are the part people lose on rather than the merits. Full page.
The five that cause the most trouble
- Medicare does not pay for long-term care. Skilled nursing after a hospital stay is short-term rehabilitation. The years of custodial care people are actually afraid of are not covered at all.
- Observation status can cost you the nursing home benefit. You can spend days in a hospital bed and still not have the inpatient stay Part A requires. Ask what your status is, out loud, before discharge.
- Original Medicare has no out-of-pocket cap. That single fact is the entire argument for buying a supplement, and it is rarely stated plainly.
- Underwriting makes the Advantage decision harder to reverse than to make. Going in is easy at any time; coming back to Medigap later may require health questions you cannot pass.
- IRMAA is a cliff on a two-year delay. One dollar moves a whole bracket, and it is charged on income from two years ago — so the planning and the pain are never in the same year.
How to use this, and how to cite it
Every term has its own anchor. Add #t- plus the term to the address and you link straight to that one definition — useful for citing a single meaning in an email, a forum answer or a document, without sending someone to a wall of text.
There are no dollar figures on this page, on purpose. Premiums, deductibles, IRMAA thresholds and the Part D cap are all reset annually. Amounts change every year; the mechanics change rarely. A glossary carrying this year’s numbers would be quietly wrong within months, and quietly wrong is worse than absent. The pages linked from each term carry the current figures and are maintained for exactly that.
Definitions here are our own plain-language writing, not copied from any agency glossary, and each term routes to the page that carries the verified detail. Where a rule has been repealed or replaced, this page says so rather than describing it in the present tense. See methodology, editorial policy and corrections — if something here is wrong or goes stale, tell us and it gets fixed with the date shown.
The two enrollment periods people mix up. AEP versus the Medicare Advantage open enrollment period, quoted from Medicare’s own table, and the mistakes that follow from mixing them up.