Fight or Flight

A primitive, physiological system evolved to enable rapid response to short term threat or stress either by fighting, fleeing or freezing via activation of the sympathetic nervous system, e.g. increased heart rate, blood pressure. Also known as acute stress response.

Reference:
Bear, M.F., Connors, B.W. & Paradiso, M.A. (2001). Neuroscience: Exploring the Brain. (2nd Edition). Baltimore, MD: Lippincott Williams & Wilkins. Cannon, W.J. (1932). The wisdom of the body. New York: Norton