Wait state
A clock cycle in which the processor does nothing but wait for slower memory to answer.
Microcontroller flash typically needs around 30 nanoseconds to deliver data, while the core wants an instruction every few nanoseconds. The gap is filled with wait states. An STM32F4 at 168 MHz needs five of them, set in a register before the clock is raised. Do it the other way round and the CPU fetches garbage and lands in a fault handler before it has configured anything else.
So that a faster core is not entirely wasted, vendors put prefetch buffers and small caches in front of the flash, or let time-critical code run from RAM. In the 1980s and 1990s, "zero wait state" was a selling point printed on PC motherboard boxes, long before most buyers knew what it meant.