phase rule

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics the rule which states that the number of degrees of freedom in a system at equilibrium equals the number of components minus the number of phases plus 2