Phase
A physically distinct, uniform state of matter — solid, liquid, or gas — set by the arrangement of its molecular bonds.
Definition
A phase is a region of matter that is uniform in its physical properties and chemical composition — most familiarly the solid, liquid, and gaseous states (plasma being a fourth, treated elsewhere). The phases differ in how their molecules are arranged and bound: rigidly ordered in a solid, mobile but touching in a liquid, and far apart and nearly independent in a gas.
Which phase a substance adopts depends on both temperature and pressure, mapped by its phase diagram. A given chemical substance can also have several solid phases (for example the many crystalline forms of ice), each a distinct phase with its own structure.
The boundaries between phases are coexistence curves along which two phases are in equilibrium; crossing one is a phase transition, accompanied by latent heat. The phase concept organises everything from melting and boiling to the exotic states near the critical and triple points.
History
The systematic treatment of phases and their equilibria was founded by Josiah Willard Gibbs in the 1870s, whose phase rule relates the number of coexisting phases to the system's degrees of freedom.