The Collaborative International Dictionary of English v.0.48:

extroversion \ex`tro*ver"sion\, n. [See Extrorse.]
   1. The condition of being turned wrong side out; as,
      extroversion of the bladder. --Dunglison.
      [1913 Webster]

   2. (Psychol.) the trait of being interested primarily in
      things in one's external environement, rather than one's
      own thoughts and feelings.
      [PJC]

   3. (Psychol.) the act of directing one's interests primarily
      toward things in one's external environement, rather than
      one's own thoughts and feelings.
      [PJC]

WordNet (r) 3.0 (2006):

extroversion
    n 1: (psychology) an extroverted disposition; concern with what
         is outside the self [syn: extraversion, extroversion]
         [ant: ambiversion, introversion]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "extroversion":
   accessibility, ambiversion, approachability, candor,
   communicativeness, complexion, conversableness,
   cycloid personality, cyclothymia, ectomorphism, ectomorphy,
   endomorphism, endomorphy, extrovertedness, frankness, freeness,
   humor, ingoingness, introversion, introvertedness, mesomorphism,
   mesomorphy, openness, other-directedness, outgoingness,
   outspokenness, personality tendency, plainness, plainspokenness,
   schizoid personality, schizothymia, sociability, somatotype,
   syntony, talkativeness, unconstraint, unrepression, unreserve,
   unreservedness, unrestraint, unrestriction, unreticence,
   unsecretiveness, unsuppression, untaciturnity