/ ɔːˈθɒrətətɪv; NAmE əˈθɔːrəteɪtɪv; əˈθɑːr-/ adjective
1.
showing that you expect people to obey and respect you :
an authoritative tone of voice
2.
that you can trust and respect as true and correct :
the most authoritative book on the subject
► au·thori·ta·tive·ly adverb