The Collaborative International Dictionary of English v.0.48:

Thermochemistry \Ther`mo*chem"is*try\, n. [Thermo- + chemistry.]
   That branch of chemical science which includes the
   investigation of the various relations existing between
   chemical action and that manifestation of force termed heat,
   or the determination of the heat evolved by, or employed in,
   chemical actions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thermochemistry
    n 1: the branch of chemistry that studies the relation between
         chemical action and the amount of heat absorbed or
         generated