The Collaborative International Dictionary of English v.0.48:

Abreaction \Ab`re*ac"tion\, n. [Pref. ab- + reaction, after G.
   Abreagirung.] (Psychotherapy)
   the purging of emotional tensions. See Catharsis, below.

   Syn: catharsis, katharsis
        [Webster 1913 Suppl. WordNet 1.5]

WordNet (r) 3.0 (2006):

abreaction
    n 1: (psychoanalysis) purging of emotional tensions [syn:
         catharsis, katharsis, abreaction]