FO4 delay
Beginner
A standard yardstick for how fast a chip’s tiny switches are, so chips made in different factories can be compared fairly.
Novice
The delay of one inverter (the simplest logic gate) driving four copies of itself. Measuring the logic in each pipeline stage in FO4 delays gives a number that stays roughly the same from one manufacturing process to the next.
Expert
A unit for architecture-level budgets: logic per stage plus register and clock overhead, both in FO4, sets the cycle time. The overhead doesn’t shrink when the logic does, so in very short stages it takes a large share of the cycle.