In supercooling, matter is in a metastable state in which a liquid of which the temperature is below the freezing point remains in the liquid state instead of solidifying.
The cause of supercooling
This state is due to very slow reaction kinetics and a need at one point for energy in order to change state. However, a small shock (providing energy) or an impurity can cause the matter to change abruptly into whatever stable form is dictated by the pressure and temperature.
A solidification nucleus in a metastable liquid. © Masakazu "Matto" Matsumoto CC by 2.0