noun Physiology, Psychology.
fight or flight reaction
The set of processes that occur in the body when it is confronted with some form of physical or mental stress. For example, if a person is faced with danger (as from a vicious animal about to attack), the nervous system signals for adrenaline and other hormones to be released into the blood. These hormones prepare the body either to confront the attacking animal or to flee to safety (thus, “fight or flight”). Changes in the body include increased heart rate, dilated pupils of the eye (to improve vision), and increased supply of blood to the muscles (to prepare the body for action).