NOUN (2)

1. a (usually irritating) impulsive and uncontrollable outburst by a problem child or a neurotic adult;

2. (psychiatry) the display of previously inhibited emotions (often in actions rather than words); considered to be healthy and therapeutic;

WordNet (r) 3.0 (2006):

acting out
    n 1: a (usually irritating) impulsive and uncontrollable
         outburst by a problem child or a neurotic adult
    2: (psychiatry) the display of previously inhibited emotions
       (often in actions rather than words); considered to be
       healthy and therapeutic