A digital system architecture, where all devices, not just memory, live in the same memory space. For example, hard disk controllers, parallel and serial UARTs, keyboard handlers and graphics chips all can be addressed using normal load and store instructions that are the same as the ones used to access normal RAM. The older, less common alternative is I/O-mapped I/O.

In terms of the hardware, a memory-mapped I/O CPU has only a single bus.

The name comes from the fact that I/O devices are "mapped" (in other words, as far as the computer is concerned, made to look like it lives in) to normal memory space.