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]