phase

A phase is a chunk of matter that is uniform all the way through: the same structure, composition, and properties everywhere inside it. Ice, liquid water, and steam are three phases of one substance. In a solid alloy, each different crystal structure is its own phase, with a real boundary where one phase meets the next.

More precisely, a phase is a homogeneous portion of a system with uniform physical and chemical characteristics. Two phases differ in structure (for example FCC versus BCC) and/or composition. Oil and water are two liquid phases because they will not mix; nickel dissolved in copper is a single FCC solid-solution phase. Cross the solubility limit and a second solid phase appears.

The whole point of a phase diagram is to tell you which phases are present, what each one's composition is, and how much of each there is at equilibrium. Properties depend enormously on the mixture: pearlite (ferrite plus cementite) is strong, while the very same steel held as all austenite is soft and non-magnetic. Counting phases is also the first step in using the Gibbs phase rule.