Definition of semi-stable state
Metastable state means that the trigger cannot reach a identifiable state within a specified period of time. When a flip-flop enters the metastable state, it cannot predict the output level of the unit, nor can it predict when the output will stabilize at a correct level. During this stabilization period, the flip-flop outputs some intermediate stage levels, or may be in an oscillating state, and this useless output level can be propagated in cascade along each flip-flop on the signal path.