substantiated or supported by documentary evidence and accepted by most authorities in a field: an authoritative edition of Shakespeare; an authoritative treatment of a subject.
3.
having an air of authority; accustomed to exercising authority; positive; peremptory; dictatorial: said with an authoritative air.
c.1600, "dictatorial" (a sense now restricted to authoritarian), from authority (q.v.). Meaning "possessing authority" is recorded from 1650s; that of "proceeding from proper authority" is from 1809.