Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Genuineness; the quality of being genuine or not corrupted
      from the original.

   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.

Source : WordNet®

authenticity
     n : undisputed credibility [syn: {genuineness}, {legitimacy}]