Metastability is the ability of a non-equilibrium state to persist for a long period of time. Usually metastability is due to a relatively slow phase transformation. For example at room temperature diamonds are metastable because the phase transformation to the stable graphite form is extremely slow. At higher temperatures the rate of phase transformation is increased and the diamond will transform to graphite.

Martensite is a metastable phase used to control the hardness of most steel. The bonds between the building blocks of polymers such as DNA, RNA and proteins are also metastable.

In electronics, the flip-flop is a device that suffers from metastability. It has two well-defined stable states, traditionally designated 0 and 1, but under certain conditions it can hover between them for longer than the normal decision time.