Definition from Wiktionary, the free dictionary
- Arising or originating from a figure of authority
- The authoritative rules in this school come not from the headmaster but from the aged matron.
- 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.
- Having a commanding style.
- He instructed us in that booming, authoritative voice of his.
Derived terms 
originating from a figure of authority
accurate or definitive
having a commanding style
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