From AMS Glossary
(Or phase transformation.) A thermodynamic process in which a substance changes from one phase to another.
A phase change entails discontinuity. At a given temperature, two phases of a pure, homogeneous substance are characterized by different enthalpies (and entropies), and the enthalpy difference is called latent heat. The phase transitions of greatest importance to meteorology are those between water vapor, liquid water, and ice.