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.
假設一個系統是由一種物質均勻組成，擁有均勻的物理與化學性質，則稱這系統只具有一種相（英語：Phase）。