- thermal (adj.)
- 1756, "having to do with hot springs," from Fr. thermal (Buffon), from Gk. therme "heat," from PIE *ghwerm-/*ghworm- "warm" (cf. L. fornax "an oven, kiln," formus "warm," O.E. wearm; see warm). Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.