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]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "thermal":
   aestival, blood-hot, blood-warm, calid, equatorial, fire storm,
   genial, hot air, luke, lukewarm, mild, room-temperature,
   subtropical, summery, sunny, sunshiny, superheated air, temperate,
   tepid, thermic, toasty, tropical, unfrozen, warm, warm as toast,
   warmish