authoritative

adjective3

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.