flight-or-fight reaction [AS, fleogan, to fly, feohtan, to fight; L, reagere, to act again] , 1. (in physiology) the reaction of the body to stress, in which the sympathetic nervous system and the adrenal medulla act to increase the cardiac output, dilate the pupils of the eyes, increase the rate of the heartbeat, constrict the blood vessels of the skin, increase the levels of glucose and fatty acids in the circulation, and induce an alert, aroused mental state. 2. (in psychiatry) a person’s reaction to stress by either fleeing from a situation or remaining and attempting to deal with it.
