The term Tg Or "glass transition temperature" is used in glasses as well as in polymers.

The Tg of a resin system defines when a thermoset polymer goes from an amorphous rigid state to a more flexible state.  Tg is an abbreviation for Glass Transition Temperature.  Tg is due to the lower heat capacity for the amorphous polymer. The Tg rubber, flexible polymer has a higher heat capacity.

For some thermoset polymers, the Tg can be below 25°C in which case the polymer will be a soft, flexible polymer at 25°C.  This is typically true with silicones as well as with flexible epoxies or urethanes.  They cure with a long flexible chain that does not require much heat to make it easy to move.  Rubbery, flexible materials at ambient temperature typically have a Tg in the 0°C to -150°C range.