1. The phase transition of a substance passing from the liquid to the solid state; solidification; the opposite of fusion. In meteorology, this almost invariably applies to the freezing of water. The phase change from the gaseous to the solid state is deposition. Like condensation, the freezing of water involves the process of nucleation. See ice point, freezing point, true freezing point, melting point. 2. Said of an environment when its temperature is equal to or less than 0°C (32°F). See freeze.