A phase is a region of space throughout which all physical properties of a material are essentially uniform. A homogeneous system is always monophasic while in a heterogeneous system the phases are the homogenous parts, of equal chemical composition and of equal physical state, mechanically separable.

Gases, since they are always completely miscible, form a single phase. On the other side, liquids can form multiple phases when they are not completely miscible with each other while solids can be marked by an unlimited number of phases.

For example, a glass containing water and oil (which remain clearly separated) is the most evident case of a system consisting of two liquid phases.

More generally, the phase is a portion of a chemical/physical system whose chemical and physical parameters (pressure, temperature, density, entropy, chemical constitution, etc.) remain constant.