metastable state or metastability

Latches and flipflops are normally considered to have two unconditionally stable operating points, either 1 or 0. However, a third operating point called a metastable state or metastability exists when the cross-coupled arrangement is exactly balanced. This operating point is stable only if there is no noise in the system and the system is perfectly balanced.

A metastable latch or flipflop has an unpredictable delay and will therefore change its output at a time that differs from the value obtained from the worst-case timing analysis. Although this delay may not present a problem in a slow system, such a delay in a fast system may lead to a failure.