[ noun ] collapse caused by exposure to excessive heat <noun.state>
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]
In New York recently, two men died of heatstroke in "Manufacturers Hanover Corporate Challenge" races; the 3.5-mile races pit employees from New York area companies against each other.