Simultaneous multithreading (SMT)
Beginner
Letting one core work on two (or more) jobs in the same tick of the clock, so gaps left by one job are filled by the other.
Novice
A core design that keeps several hardware threads and issues instructions from more than one of them in the same clock cycle, filling execution slots that one thread would leave empty. Intel’s brand name is Hyper-Threading.
Expert
Fills both vertical waste (cycles with nothing to issue, e.g. on a miss) and horizontal waste (part-filled cycles). Raises throughput by tens of percent at the cost of per-thread speed, shared-cache contention and weaker isolation between threads.
Explained in Multicore and vector units (Architectures).
See also: Hardware thread (logical processor), ILP wall.