phase rule

WordReference Random House Unabridged Dictionary of American English © 2026

phase′ rule′, [Physical Chem.]

1. Chemistry a law that the number of degrees of freedom in a system in equilibrium is equal to two plus the number of components less the number of phases. Thus, a system of ice, melted ice, and water vapor, being one component and three phases, has no degrees of freedom. Cf. variance (def. 4).

• 1895–1900

Collins Concise English Dictionary © HarperCollins Publishers::

phase rule noun

1. the principle that in any system in equilibrium the number of degrees of freedom is equal to the number of components less the number of phases plus two
   See also degree of freedom, component4