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From The Collaborative International Dictionary of English v.0.48:
Thermal \Ther"mal\, a. [L. thermae hot springs, fr. Gr. ?, pl. of ? heat, fr. ? hot, warm, ? to warm, make hot; perhaps akin to L. formus warm, and E. forceps.] 1. Of or pertaining to heat; warm; hot; as, the thermal unit; thermal waters. [1913 Webster] The thermal condition of the earth. --J. D. Forbes. [1913 Webster] 2. caused by or affected by heat; as, thermal springs. [PJC] 3. designed to retain heat; as, thermal underwear. [PJC] Thermal conductivity, Thermal spectrum. See under Conductivity, and Spectrum. Thermal unit (Physics), a unit chosen for the comparison or calculation of quantities of heat. The unit most commonly employed is the amount of heat necessary to raise the temperature of one gram or one pound of water from zero to one degree Centigrade. See Calorie, and under Unit. [1913 Webster]