The Collaborative International Dictionary of English v.0.48:

Authenticate \Au*then"ti*cate\, v. t. [imp. & p. p.
   Authenticated; p. pr. & vb. n. Authenticating (?).] [Cf.
   LL. authenticare.]
   1. To render authentic; to give authority to, by the proof,
      attestation, or formalities required by law, or sufficient
      to entitle to credit.
      [1913 Webster]

            The king serves only as a notary to authenticate the
            choice of judges.                     --Burke.
      [1913 Webster]

   2. To prove authentic; to determine as real and true; as, to
      authenticate a portrait. --Walpole.
      [1913 Webster]

WordNet (r) 3.0 (2006):

authenticate
    v 1: establish the authenticity of something

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "authenticate":
   OK, accept, accredit, affirm, amen, approve, attest, authorize,
   autograph, avouch, back, back up, bear out, bolster, buttress,
   certify, circumstantiate, confirm, corroborate, cosign,
   countersign, demonstrate, document, endorse, fortify,
   give permission, give the go-ahead, give the imprimatur,
   give thumbs up, initial, justify, notarize, pass, pass on,
   pass upon, permit, probate, prove, ratify, reinforce, rubber stamp,
   sanction, say amen to, seal, second, sign, sign and seal,
   strengthen, subscribe to, substantiate, support, sustain,
   swear and affirm, swear to, test, try, undergird, undersign,
   underwrite, uphold, validate, verify, visa, vise, vouch for,
   warrant