The states in which matter can exist: as a solid, liquid, or gas. When temperature changes, matter can undergo a phase change, shifting from one form to another. Examples of phase changes are melting (changing from a solid to a liquid), freezing (changing from a liquid to a solid), evaporation (changing from a liquid to a gas), and condensation (changing from a gas to a liquid). (See boiling point, condensation point, freezing point, melting point, triple point, and vaporization.)