From Glossary of Meteorology
(Also called subcooling or undercooling; see note below.) The reduction of temperature of any liquid below the melting point of that substance's solid phase; that is, cooling beyond its nominal freezing point.
A liquid may be supercooled to varying degrees, depending upon the relative lack of freezing nuclei or solid boundary irregularities within its environment, and freedom from agitation. This supercoolability of a substance with a crystalline solid form (as opposed to amorphous matter) stems from the unique energy transformations necessary for the formation of the first crystal nucleus, whereafter all adjacent liquid immediately becomes solid unless or until the latent heat released elevates the system's temperature sufficiently to arrest the process. It should be noted that the reverse process is not possible; a crystalline solid cannot be "superheated"; therefore, a substance's melting point is very conservative, only slightly dependent upon pressure. Supercooled clouds are quite common. In extreme cases, they have been observed at temperatures as low as -40°C (-40°F). The smaller and purer the water droplets, the more likely is supercooling.
(Note: The choice of terminology for this concept has been variable and controversial for many years, largely based on conceptual views of the prefixes super, sub, and under. Supercooling remains, however, the most frequently utilized term.)