phase rule

an expression of the relationships existing between systems in equilibrium: P + V = C + 2, where P is the number of phases, V the variance or degrees of freedom, and C the number of components; it also follows that the variance is V = C + 2 - P. For H₂O at its triple point, V = 1 + 2 - 3 = 0, that is, both temperature and pressure are fixed.

Synonym(s): Gibbs phase rule