Authoritative, a.
1. Having, or proceeding from, due authority; entitled to obedience, credit, or acceptance; determinate; commanding. The sacred functions of authoritative teaching. Barrow.
2. Having an air of authority; positive; dictatorial; peremptory; as, an authoritative tone. The mock authoritative manner of the one, and the insipid mirth of the other. Swift. -- Au*thor"i*ta*tive*ly, adv -- Au*thor"i*ta*tive*ness, n.
Shades of meaning
Fernald has no entry of its own for authoritative; he lists it under absolute, authentic, dogmatic.
Where he writes about it:
Authoritative is used always in a good sense, implying the right to claim authority; imperative, peremptory, and positive are used ordinarily in the good sense; as, an authoritative definition; an imperative demand; a peremptory command; positive instructions; imperious signifies assuming and determined to command, rigorously requiring obedience. — from absolute
Synonyms
absoluteauthenticdogmaticacceptedaccreditedarbitraryarrogantauthorizedautocraticcertaincoercivecommandingcompulsivecompulsorycontrollingcurrentdespoticdictatorialdoctrinaldomineeringexactinggenuinehaughtyimperativeimperiousirresponsiblelegitimatelordlymagisterialopinionatedoriginaloverbearingperemptorypositiverealreceivedreliableself-opinionatedsupremesuresystematictruetrustworthytyrannicalunconditionalunequivocalveritable