Transition temperature
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.