The Collaborative International Dictionary of English v.0.48:

Apostasy \A*pos"ta*sy\, n.; pl. Apostasies. [OE. apostasie, F.
   apostasie, L. apostasia, fr. Gr. ? a standing off from, a
   defection, fr. ? to stand off, revolt; ? from + ? to stand.
   See Off and Stand.]
   An abandonment of what one has voluntarily professed; a total
   desertion of departure from one's faith, principles, or
   party; esp., the renunciation of a religious faith; as,
   Julian's apostasy from Christianity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

apostasy
    n 1: the state of having rejected your religious beliefs or your
         political party or a cause (often in favor of opposing
         beliefs or causes) [syn: apostasy, renunciation,
         defection]
    2: the act of abandoning a party for cause [syn: apostasy,
       tergiversation]