The Collaborative International Dictionary of English v.0.48:

Phase rule \Phase rule\ (Phys. Chem.)
   A generalization with regard to systems of chemical
   equilibrium, discovered by Prof. J. Willard Gibbs. It may be
   stated thus: The degree of variableness (number of degrees of
   freedom) of a system is equal to the number of components
   minus the number of phases, plus two. Thus, if the components
   be salt and water, and the phases salt, ice, saturated
   solution, and vapor, the system is invariant, that is, there
   is only one set of conditions under which these four phases
   can exist in equilibrium. If only three phases be considered,
   the system is univariant, that is, the fixing of one
   condition, as temperature, determines the others.
   [Webster 1913 Suppl.]