A phase is a region of matter that is macroscopically uniform in composition and physical state. Within one phase, properties vary smoothly rather than jumping abruptly from one material state to another.

Solid, liquid and gas can form different phases, but phase is more general than those three labels. Two different solid structures can be separate phases, and two immiscible liquids can form distinct liquid phases.

A sample can contain one phase or several phases at the same time. Liquid water in equilibrium with water vapor contains two phases of the same chemical substance. Oil and water form two liquid phases because they remain separated into regions with different composition and properties.

The boundary between two phases is an interface. Properties such as composition, density or structure can change across that boundary.

A homogeneous sample contains a single uniform phase at the scale of interest. A heterogeneous sample contains distinguishable regions belonging to different phases.

Phase is therefore a macroscopic description: it groups together regions that can be treated as physically uniform for the question being studied. Later models explain when phases coexist, how phase boundaries move and how temperature, pressure and composition determine which phases are stable.