The Collaborative International Dictionary of English v.0.48:

Authenticity \Au`then*tic"i*ty\, n. [Cf. F. authenticit['e].]
   1. The quality of being authentic or of established authority
      for truth and correctness.
      [1913 Webster]

   2. Genuineness; the quality of being genuine or not corrupted
      from the original.
      [1913 Webster]

   Note: In later writers, especially those on the evidences of
         Christianity, authenticity is often restricted in its
         use to the first of the above meanings, and
         distinguished from qenuineness.
         [1913 Webster]

WordNet (r) 3.0 (2006):

authenticity
    n 1: undisputed credibility [syn: authenticity, genuineness,
         legitimacy]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "authenticity":
   absolute realism, accomplished fact, actuality, artlessness,
   authoritativeness, bona fideness, calculability, canonicalness,
   canonicity, creativeness, creativity, dependability, factuality,
   fait accompli, faithworthiness, firmness, freshness, genuineness,
   gospel truth, grim reality, historicity, honesty, inartificiality,
   innovation, inventiveness, invincibility, legitimacy, lifelikeness,
   literalism, literality, literalness, naturalism, naturalness,
   newness, nonimitation, not a dream, novelty, objective existence,
   originality, orthodoxicalness, orthodoxism, orthodoxness,
   orthodoxy, photographic realism, predictability, realism, reality,
   realness, reliability, religious truth, right belief, rightness,
   secureness, security, sincerity, solidity, soundness, stability,
   staunchness, steadfastness, steadiness, substantiality, the truth,
   traditionalism, true-to-lifeness, trustworthiness, truth,
   truth to nature, unadulteration, unaffectedness, unerringness,
   unfictitiousness, uniqueness, unspeciousness, unspuriousness,
   unsyntheticness, validity, verisimilitude