“Acting out: 1. In psychoanalysis, the enactment rather than the recollection of past events, especially enactments relating to the transference during therapy. It is often impulsive and aggressive, and it is usually uncharacteristic of the patient’s normal behavior. The concept was introduced by Sigmund Freud (1856-1839) in An Outline of Psychoanalysis (1938/40): the patient ‘acts it [the past event] before us, as it were, rather than reporting it to us’ (Standard Edition, XXIII, pp. 144-207, at p. 176). 2. A defense mechanism in which unconscious emotional conflicts or impulses are dealt with by actions, including parapraxes, rather than thought or contemplation. act out vb.”
Excerpted from: Colman, Andrew M., ed. Oxford Dictionary of Psychology. New York: Oxford University Press, 2003.