Hyperthermia: Overheating of the body, possibly due to extreme weather conditions. Unrelieved hyperthermia can lead to collapse and death, particularly in the elderly. Hyperthermia can be prevented in many cases through use of air conditioning and ventilation, as well as by ensuring that vulnerable persons drink extra water. In emergency cases of hyperthermia, injections of saline solution and rapid cooling of the body may be necessary. Also known as heatstroke and heat prostration.