Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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: {renunciation}, {defection}]
     2: the act of abandoning a party or cause [syn: {tergiversation}]