The Collaborative International Dictionary of English v.0.48:

ambiversion \ambiversion\ n.
   1. a balanced disposition intermediate between extroversion
      and introversion.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

ambiversion
    n 1: (psychology) a balanced disposition intermediate between
         extroversion and introversion [ant: extraversion,
         extroversion, introversion]