Clock
Beginner
A steady tick signal that tells every memory cell on the chip when to update, like a metronome.
Novice
A signal that switches between 0 and 1 at a steady rate, for example a billion times a second (1 GHz). Each rise from 0 to 1 is a clock edge, the moment flip-flops take in new values. The time between edges is the clock period.
Expert
The time reference of a synchronous design. Every path between flip-flops must settle within one period, minus margins, so the slowest path sets the highest usable clock frequency. Chips usually have several clocks, which divides them into clock domains.