auth

a. [ OE. autentik, OF. autentique, F. authentique, L. authenticus coming from the real author, of original or firsthand authority, from Gr. &unr_;, fr. &unr_; 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. [1913 Webster]

4. (Law) Vested with all due formalities, and legally attested. [1913 Webster]

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. These words, as here compared, have reference to historical documents.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]

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]