The Collaborative International Dictionary of English v.0.48:

heatstroke \heatstroke\ n.
   A physiological disturbance caused by exposure to excessive
   heat, resulting in rapid pulse, hot dry skin, and fever,
   leading to loss of consciousness.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

heatstroke
    n 1: collapse caused by exposure to excessive heat [syn:
         heatstroke, heat hyperpyrexia]