Heat stroke, also known as sun stroke, is a severe heat illness, defined as hyperthermia with a body temperature greater than 40.6 °C (105.1 °F) because of environmental heat exposure with lack of thermoregulation. This is distinct from a fever, where there is a physiological increase in the temperature set point of the body. The term "stroke" in "heat stroke" is a misnomer in that it does not involve a blockage or hemorrhage of blood flow to the brain. Preventive measures include drinking plenty of cool liquids and avoiding excessive heat and humidity, especially in unventilated spaces, such as parked cars, that can overheat quickly. Treatment requires rapid physical cooling of the body.