The transition that occurs when a liquid is cooled very rapidly to become a glass. The glass transition temperature of a material is not a clear-cut temperature like the melting point. This is the case because the glassy state is a metastable state, with the transition temperature becoming lower as the rate of cooling decreases. The freezing temperature for a material is always higher than the glass transition temperature for that material. The theory of the glass transition is much more complicated than the theory of normal phase transitions.