worthy of belief as conforming to fact or reality; trustworthy
an authentic account
not imaginary, false, or imitation; genuine
The signature looked authentic enough
said of a mode in church music: ranging upward from the keynote1 (the note on which the mode is based): compare plagal
authentically adv
authenticity /-ʹtisiti/ noun
[Middle English autentik via Old French from Greek authentikos from authentēs perpetrator, master]