Wait state

A wait state is a delay experienced by a computer processor when accessing external memory or another device that is slow to respond.

As of late 2011, computer microprocessors run at very high speeds, while memory technology does not seem to be able to catch up: typical PC processors like the Intel Core 2 and the AMD Athlon 64 X2 run with a clock of several GHz, which means that one clock cycle is less than 1 nanosecond (typically about 0.3 ns to 0.5 ns on modern desktop CPUs), while main memory has a latency of about 15–30 ns. Some second-level CPU caches run slower than the processor core.