Synonyms for Authoritative (same or very similar meaning)
WordNet sense 1 (of recognized authority or excellence):
classical, definitive
WordNet sense 2 (established or widely recognized as a model of authority or excellence):
standard
WordNet sense 3 (having authority or ascendancy or influence):
important
WordNet sense 4 (sanctioned by established authority):
authorised, authorized
WordNet sense 5 (having official authority or sanction):
official
From the ODE community, based on WordNet
add/edit