The Collaborative International Dictionary of English v.0.48:

Sunstroke \Sun"stroke`\, n. (Med.)
   Any affection produced by the action of the sun on some part
   of the body; especially, a sudden prostration of the physical
   powers, with symptoms resembling those of apoplexy,
   occasioned by exposure to excessive heat, and often
   terminating fatally; coup de soleil.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sunstroke
    n 1: sudden prostration due to exposure to the sun or excessive
         heat [syn: sunstroke, insolation, thermic fever,
         siriasis]