MAPD stands for Medicare Advantage Prescription Drug. An MAPD plan is a Medicare Advantage plan that already has Part D prescription drug coverage built into it. One plan, one ID card, one premium, one company to call.
The short version. MA is Medicare Advantage, your Part A and Part B benefits delivered through a private plan. PD is prescription drugs, the Part D coverage. Put them together in one policy and you have an MAPD.
The alternative is called an MA-only plan: the same Medicare Advantage medical coverage with no drug coverage attached. Those exist mainly for people who get drug coverage somewhere else, such as through the VA or a former employer. Most Medicare Advantage plans sold to individuals are MAPD plans, which is why the two terms get used almost interchangeably even though they are not the same thing.
One rule catches people out: if you enrol in an MA-only plan, you generally cannot buy a standalone Part D plan to sit alongside it. Doing so will usually disenroll you from the Advantage plan. If you want drug coverage and a Medicare Advantage plan, you want an MAPD.