The Collaborative International Dictionary of English v.0.48:

Unfaithful \Un*faith"ful\, a.
   1. Not faithful; not observant of promises, vows, allegiance,
      or duty; violating trust or confidence; treacherous;
      perfidious; as, an unfaithful subject; an unfaithful agent
      or servant.
      [1913 Webster]

            My feet, through wine, unfaithful to their weight.
                                                  --Pope.
      [1913 Webster]

            His honor rooted in dishonor stood,
            And faith unfaithful kept him falsely true.
                                                  --Tennyson.
      [1913 Webster]

   2. Not possessing faith; infidel. [R.] --Milton.
      [1913 Webster] -- Un*faith"ful*ly, adv. --
      Un*faith"ful*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "unfaithfulness":
   Punic faith, adulterous affair, adultery, affair, amour, bad faith,
   barratry, breach of faith, breach of promise, breach of trust,
   cuckoldry, dereliction, disaffection, disloyalty, entanglement,
   eternal triangle, faithlessness, falseness, falsity, fickleness,
   flirtation, forbidden love, hanky-panky, illicit love, inconstancy,
   infidelity, intrigue, liaison, love affair, mala fides,
   perfidiousness, perfidy, recreancy, romance, romantic tie,
   triangle, trothlessness, unfaith, unloyalty, unsteadfastness,
   untrueness