The point at which a substance changes from one state of aggregation to another. This general definition would include the melting point (transition from solid to liquid), boiling point (liquid to gas), or sublimation point (solid to gas); but in practice the term transition point is usually restricted to the transition from one solid phase to another, that is, the temperature (for a fixed pressure, usually 1 atm or 100 kilopascals) at which a substance changes from one crystal structure to another.