Source : Webster's Revised Unabridged Dictionary (1913)

Exothermic \Ex`o*ther"mic\, a. [Pref. exo- + thermic.] (Chem.)
   Characterized by, or formed with, evolution of heat; as, an
   exothermic reaction; -- opposed to {endothermic}.

Source : WordNet®

exothermic
     adj : (of a chemical reaction or compound) occurring or formed
           with evolution of heat [syn: {exothermal}, {heat-releasing}]
           [ant: {endothermic}]