heat exhaustion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A condition caused by exposure to heat, resulting in the depletion of body fluids and causing weakness, dizziness, nausea, and often collapse. The condition can be alleviated by rest and the administration of fluids and electrolytes to compensate for those lost through excessive sweating.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a condition marked by dizziness and nausea and weakness caused by depletion of body fluids and electrolytes