Glass-liquid transition temperature, which is approximately where plastics and other materials change from hard and relatively brittle into flexible and rubbery.

As the other comments here noted, it doesn’t exactly mean that the material is safe to use for a rigid part below that temperature, and the transition extends over a range in temperatures, but it does give you a rough idea about the behavior of a material at various temperatures.

Glass transition temperature.

https://en.wikipedia.org/wiki/Glass_transition

Glass transition temperature I think