1. Arising or originating from a figure of authority
    The authoritative rules in this school come not from the headmaster but from the aged matron.
  2. Highly accurate or definitive; treated or worthy of treatment as a scholarly authority
    This book is the world's most authoritative guide to insect breeding habits.
  3. Having a commanding style.
    He instructed us in that booming, authoritative voice of his.

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