The Collaborative International Dictionary of English v.0.48:

Authentic \Au*then"tic\, a. [OE. autentik, OF. autentique, F.
   authentique, L. authenticus coming from the real author, of
   original or firsthand authority, from Gr. ?, fr. ? suicide, a
   perpetrator or real author of any act, an absolute master;
   a'yto`s self + a form "enths (not found), akin to L. sons and
   perh. orig. from the p. pr. of e'i^nai to be, root as, and
   meaning the one it really is. See Am, Sin, n., and cf.
   Effendi.]
   1. Having a genuine original or authority, in opposition to
      that which is false, fictitious, counterfeit, or
      apocryphal; being what it purports to be; genuine; not of
      doubtful origin; real; as, an authentic paper or register.
      [1913 Webster]

            To be avenged
            On him who had stole Jove's authentic fire.
                                                  --Milton.
      [1913 Webster]

   2. Authoritative. [Obs.] --Milton.
      [1913 Webster]

   3. Of approved authority; true; trustworthy; credible; as, an
      authentic writer; an authentic portrait; authentic
      information.
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   4. (Law) Vested with all due formalities, and legally
      attested.
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   5. (Mus.) Having as immediate relation to the tonic, in
      distinction from plagal, which has a correspondent
      relation to the dominant in the octave below the tonic.
      [1913 Webster]

   Syn: Authentic, Genuine.

   Usage: These words, as here compared, have reference to
          historical documents. We call a document genuine when
          it can be traced back ultimately to the author or
          authors from whom it professes to emanate. Hence, the
          word has the meaning, "not changed from the original,
          uncorrupted, unadulterated:" as, a genuine text. We
          call a document authentic when, on the ground of its
          being thus traced back, it may be relied on as true
          and authoritative (from the primary sense of "having
          an author, vouched for"); hence its extended
          signification, in general literature, of trustworthy,
          as resting on unquestionable authority or evidence;
          as, an authentic history; an authentic report of
          facts.
          [1913 Webster]

                A genuine book is that which was written by the
                person whose name it bears, as the author of it.
                An authentic book is that which relates matters
                of fact as they really happened. A book may be
                genuine without being, authentic, and a book may
                be authentic without being genuine. --Bp.
                                                  Watson.
          [1913 Webster]

   Note: It may be said, however, that some writers use
         authentic (as, an authentic document) in the sense of
         "produced by its professed author, not counterfeit."
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Authentic \Au*then"tic\, n.
   An original (book or document). [Obs.] "Authentics and
   transcripts." --Fuller.
   [1913 Webster]

WordNet (r) 3.0 (2006):

authentic
    adj 1: conforming to fact and therefore worthy of belief; "an
           authentic account by an eyewitness"; "reliable
           information" [syn: authentic, reliable]
    2: not counterfeit or copied; "an authentic signature"; "a bona
       fide manuscript"; "an unquestionable antique"; "photographs
       taken in a veritable bull ring" [syn: authentic, bona
       fide, unquestionable, veritable]

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "authentic":
   Christian, aboveboard, absolute, accepted, accurate, actual,
   adducible, admissible, approved, attestative, attestive,
   authoritative, avant-garde, based on, bona fide, candid, canonical,
   card-carrying, cathedral, certain, circumstantial, cognizable,
   conclusive, conventional, convincing, correct, creative, credible,
   cumulative, customary, damning, de facto, decisive, dependable,
   determinative, dinkum, documentary, documented, evangelical,
   evidential, evidentiary, ex cathedra, ex parte, eye-witness,
   factual, fair and square, faithful, final, firm, firsthand,
   following the letter, for real, founded on, foursquare, fresh,
   genuine, good, good-faith, grounded on, hearsay, historical,
   honest, honest-to-God, imaginative, implicit, inartificial,
   incontrovertible, indicative, indisputable, indubitable,
   irrefutable, irresistible, knowable, lawful, legitimate, lifelike,
   literal, magisterial, material, natural, naturalistic, new, novel,
   nuncupative, of the faith, official, on the level, on the square,
   on the up-and-up, open, open and aboveboard, original, orthodox,
   orthodoxical, overwhelming, positive, presumptive, probative,
   proper, pukka, pure, questionless, real, realistic, received,
   recognizable, reliable, revolutionary, right, rightful, scriptural,
   significant, simon-pure, simple, sincere, single-hearted, solid,
   sound, square, square-dealing, square-shooting, standard, sterling,
   straight, straight-shooting, substantial, suggestive, sure,
   sure-enough, symptomatic, telling, textual, traditional,
   traditionalistic, true, true to life, true to nature,
   true to reality, true-blue, trustworthy, trusty, unadulterated,
   unaffected, unalloyed, unassumed, unassuming, uncolored,
   unconcocted, uncopied, uncounterfeited, undeniable, underived,
   undisguised, undisguising, undisputed, undistorted, undoubted,
   unexaggerated, unfabricated, unfanciful, unfeigned, unfeigning,
   unfictitious, unflattering, unimagined, unimitated, uninvented,
   unique, unpretended, unpretending, unqualified, unquestionable,
   unromantic, unsimulated, unspecious, unsynthetic, unvarnished,
   up-and-up, valid, verbal, verbatim, veridical, verisimilar,
   veritable, very, weighty, word-for-word