(physical chemistry) a distinct state of matter in a system; matter that is identical in chemical composition and physical state and separated from other material by the phase boundary WordNet 3.0 & Open English WordNet

In the physical sciences, a phase is a region of material that is chemically uniform, physically distinct, and mechanically separable. Wikipedia

A region of space throughout which all physical properties are essentially uniform Wikipedia Disambiguation

Region in a substance throughout which all physical properties are essentially uniform; region of material that is chemically uniform, physically distinct, mechanically separable Wikidata

A component in a material system that is distinguished by chemical composition and/or physical state (solid, liquid or gas) and/or crystal structure. It is delineated from an adjoining phase by an abrupt change in one or more of those conditions. Wiktionary

Chemistry: component in a material system. Wiktionary (translation)