A temperature below which an amorphous material becomes substantially less ductile. It may be above or below room temperature.
In practice, the glass transition phenomenon happens over a relatively narrow temperature range, and the glass transition temperature is the midpoint of that range. The width of the glass transition range varies with material, and is seldom reported.
It is generally best to avoid using a material the glass transition temperature of which is within the expected operating temperature range. Some thermoplastics that are unusually brittle at room temperature are that way because their glass transition temperature is above room temperature, while that of most thermoplastics is below.
The glass transition in rubber can be used to machine rubber parts after cooling them to a rigid (and therefore easier to cut precisely) state. The transition is fully reversible without damage to the material.
There is a large number of ASTM testing standards for this property, including D3418, D6604, E1356, E1545, E1640, and E1824. ISO standards include 11357-2 and 11359-2.