The Tg of epoxy is the Glass Transition temperature. This is the general temperature range when the epoxy goes from a solid, rigid state to a rubbery state, while the molecules are still cross-linked. The Tg is used to understand other properties of an epoxy at room temperature, including compression, tension, and shear.
As opposed to Tg, HDT (Heat Deflection Temperature) is the temperature when an epoxy deforms under a load, which is commonly considered a more valid, functionally relevant number.