The Collaborative International Dictionary of English v.0.48:

Infidelity \In`fi*del"i*ty\, n.; pl. Infidelities. [L.
   infidelitas: cf. F. infid['e]lit['e].]
   [1913 Webster]
   1. Lack of faith or belief in some religious system;
      especially, a lack of faith in, or disbelief of, the
      inspiration of the Scriptures, of the divine origin of
      Christianity.
      [1913 Webster]

            There is, indeed, no doubt but that vanity is one of
            the principal causes of infidelity.   --V. Knox.
      [1913 Webster]

   2. Unfaithfulness to the marriage vow or contract; violation
      of the marriage covenant by adultery.
      [1913 Webster]

   3. Breach of trust; unfaithfulness to a charge, or to moral
      obligation; treachery; deceit; as, the infidelity of a
      servant. "The infidelity of friends." --Sir W. Temple.
      [1913 Webster]

WordNet (r) 3.0 (2006):

infidelity
    n 1: the quality of being unfaithful [syn: infidelity,
         unfaithfulness] [ant: faithfulness, fidelity]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "infidelity":
   Punic faith, adulterous affair, adultery, affair, agnosticism,
   amor, amour, apostasy, atheism, bad faith, barratry,
   breach of faith, breach of promise, breach of trust, cheating,
   cuckoldry, denial, dereliction, disaffection, disbelief, discredit,
   disloyalty, entanglement, eternal triangle, faithlessness,
   falseness, falsity, fickleness, flirtation, forbidden love,
   gentilism, hanky-panky, heresy, illicit love, inability to believe,
   inconstancy, incredulity, intrigue, liaison, love affair,
   mala fides, minimifidianism, misbelief, nonbelief, nullifidianism,
   perfidy, recreancy, rejection, romance, romantic tie, secularism,
   traitorousness, treachery, triangle, trothlessness, unbelief,
   unbelievingness, unfaith, unfaithfulness, unloyalty,
   unsteadfastness, untrueness