Plain English Meaning
When the body gets so overheated that it can no longer cool itself down, and the person becomes seriously ill. Without fast cooling and medical help, it can kill.
Full Definition
A serious medical condition in which the body's temperature regulation fails after prolonged exposure to high heat, causing core body temperature to rise to dangerous levels. Symptoms include hot dry skin, a rapid and strong pulse, headache, confusion, nausea, and eventually collapse or unconsciousness. Heatstroke can be fatal if not treated quickly.
Context Anchor
In flight training, this term appears in human factors discussions about hot weather, long preflights, cockpit heat, and not having enough water in the body.