If it has to do with heat, it’s thermal. Wearing a thermal shirt under your sweater helps you stay warm on a brutally cold day. Your thermal coffee mug keeps your coffee hot.
The Greek word therme, meaning “heat,” is the origin of the adjective thermal. Something that is thermal is hot, retains heat, or has a warming effect. If your sweatshirt has a thermal lining, its texture might remind you of a waffle-that's what traps your body heat. Also, there are large currents of warm air that flow upwards — these are what gliders or big birds like hawks ride to stay afloat — one type of those currents is called a “thermal.”