Flip-flop (register)
Beginner
A tiny memory that holds one bit and only changes when the clock ticks.
Novice
A circuit that stores one bit (a 0 or a 1). It copies its input to its output only at the instant the clock rises, and holds that value until the next rise. A group of flip-flops that hold a number together is a register.
Expert
The storage element of a clocked design. Its input must be steady for a short window around the clock edge (the setup time before, the hold time after); a change inside that window can leave it metastable. RTL implies a flop wherever a clocked block assigns a value, and cell libraries offer versions with reset, enable and test (scan) inputs.
Explained in RTL design (Design Flow).
See also: Register-transfer level (RTL), Clock, Metastability.