Metastability
Beginner
A moment when a memory cell can’t decide between 0 and 1 because its input changed just as the clock ticked.
Novice
An undecided state between 0 and 1 that a flip-flop can enter when its input changes at almost the same instant as the clock edge. It settles to 0 or 1 eventually, usually within a fraction of a nanosecond, but with no guaranteed limit.
Expert
The time to settle is exponentially distributed with a time constant (tau): the chance a flop is still undecided after waiting is . Each extra of waiting divides the failure rate by (about 2.7), and synchronizer design rests on that.
Explained in RTL design (Design Flow).
See also: Synchronizer (two-flop), MTBF (synchronizer).