The instability associated with a supercooled or supersaturated phase of matter. A small fluctuation (on a molecular scale) or foreign particle may cause change to a more stable phase. For example, at a temperature below the equilibrium melting point of ice, the ice phase is stable, whereas liquid water is supercooled and is unstable because its chemical potential is higher than that of ice. A small fluctuation such as collision with ice may cause the supercooled water to change into ice.