I. ther‧mal¹ /ˈθɜːməl $ ˈθɜːr-/ adjective [only before noun]
[Date: 1700-1800; Language: Greek; Origin: therme; ⇒ therm]

1. relating to or caused by heat:
thermal energy

2. thermal clothing is made from special material to keep you warm in very cold weather:
thermal underwear

3. thermal water is heated naturally under the earth:
thermal springs

[TahlilGaran] Dictionary of Contemporary English ▲

II. thermal² noun

1. [countable] a rising current of warm air used by birds

2. thermals [plural] British English informal special warm clothing, especially underwear, worn under your other clothes

[TahlilGaran] Dictionary of Contemporary English ▲