(1) Of, relating to, or caused by heat. (2) Designed to insulate in order to retain body heat.
Example:
The glider circled slowly, seeking a thermal updraft from a plowed field that would take it spiraling upward.
Before polypropylene and thermal weave, union suits -- that is, long thermal underwear that covered the entire body -- were sometimes donned in October and not taken off until April. Worn by sodbusters, cowboys, and townsfolk alike, they kept America warm during its formative years. They undoubtedly also kept America itchy and a little on the smelly side through the cold months. But then, bathing even once a week was considered the height of cleanliness until very recently.