Source : Webster's Revised Unabridged Dictionary (1913)

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.]
   Of or pertaining to heat; warm; hot; as, the thermal unit;
   thermal waters.

         The thermal condition of the earth.      --J. D.
                                                  Forbes.

   {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}.

Source : WordNet®

thermal
     adj 1: relating to or associated with heat; "thermal movements of
            molecules"; "thermal capacity"; "thermic energy"; "the
            caloric effect of sunlight" [syn: {thermic}, {caloric}]
            [ant: {nonthermal}]
     2: of or relating to hot a hot spring; "thermal water"
     3: caused by or designed to retain heat; "a thermal burn";
        "thermal underwear"

thermal
     n : rising current of warm air