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]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "sunstroke":
   Minamata disease, altitude sickness, anoxemia, anoxia,
   anoxic anoxia, anthrax, black lung, caisson disease, calenture,
   chilblain, coup de soleil, decompression sickness, frostbite,
   heat prostration, immersion foot, insolation, itai, jet lag,
   lead poisoning, mercury poisoning, motion sickness, pneumoconiosis,
   radiation sickness, radionecrosis, red-out, the bends,
   thermic fever, trench foot