Real Dictionary

heat exhaustion in American English
a condition caused by excessive loss of salt and water, usually resulting from overexertion in a hot environment, and characterized by peripheral vascular collapse, dizziness, nausea, low body temperature, clammy skin, etc.
noun: a condition characterized by faintness, rapid pulse, nausea, profuse sweating, cool skin, and collapse, caused by prolonged exposure to heat accompanied by loss of adequate fluid and salt from the body

heat exhaustion in British English
noun: a condition resulting from exposure to intense heat, characterized by dizziness, abdominal cramp, and prostration