Wiktionary
n. 1 (context physics English) The temperature at which one phase of a material changes into another; a transition point 2 (context physics chemistry English) The temperature at which one form of a polymorph changes into another
Wikipedia
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.
Usage examples of "transition temperature".
The new compounds have unlimited current density capacities, and remain superconductive up to the antiferromagnetic transition temperature, in the area of 330 degrees Celsius.