From Middle English authentik, from Old French autentique, from Latin authenticus, from Ancient Greek αὐθεντικός, from Ancient Greek αὐθέντης.

• (British) IPA: /ɒ.ˈθɛn.tɪk/
• (America) IPA: /ɔ.ˈθɛn.tɪk/
• (cot-caught) IPA: /ɑ.ˈθɛn.tɪk/

authentic

1. Of the same origin as claimed; genuine.

   The experts confirmed it was an authentic signature.

2. Conforming to reality and therefore worthy of trust, reliance, or belief.

   an authentic writer; an authentic portrait; authentic information

   The report was completely authentic.

3. (music)
   1. Designating a mode having the final as the lowest note.
   2. Designating a cadence in which the dominant chord precedes the tonic.
4. (obsolete) Authoritative.
   • 1641 June or July, John Milton, Of Prelatical Episcopacy, and Whether It may be Deduc'd from the Apostolical Times by Virtue of Those Testimonies which are Alledg'd to that Purpose in Some Late Treatises; […]; republished in A Complete Collection of the Historical, Political, and Miscellaneous Works of John Milton, […], volume I, Amsterdam [actually London: s.n.], 1698, →OCLC ↗, page 242 ↗:

     [T]here they find Ignatius, and then they believe him not for his ovvn Authority, but for a truths ſake, vvhich they derive from elſvvhere: to vvhat end then ſhould they cite him as Authentick for Epiſcopacy, vvhen they cannot knovv vvhat is Authentick in him, but by the judgment vvhich they brought vvith them, and not by any judgment vvhich they might ſafely learn from him?

• (of the claimed origin) genuine, real, bonafide, bona fide, unfaked
• (conforming to fact) reliable, trustworthy, credible, unfaked

• (antonym(s) of “not of the claimed origin”): non-authentic, nonauthentic, phony, fake; ingenuine, inauthentic, unauthentic

• French: authentique
• German: authentisch, echt
• Italian: autentico
• Portuguese: autêntico
• Russian: по́длинный
• Spanish: auténtico

• German: authentisch, echt, wahr
• Portuguese: autêntico
• Russian: достове́рный