.

Transition temperature is the temperature at which a material changes from one crystal state (allotrope) to another. For example, when rhombic sulfur is heated above 96°C it changes form into monoclinic sulfur. When cooled below 96°C it reverts to rhombic sulfur.

In the case of ferroelectric or ferromagnetic crystals a transition temperature may be known as the Curie temperature.

The term is also used for the temperature at which a change of phase occurs or at which a substance becomes superconducting.