authoritative

Authoritative describes someone or something having power, influence or the right to control and make decisions. (adjective)

When a parent speaks to a child in a certain tone of voice so that the child knows he must obey, that is an example of an authoritative voice.
A supervisor who is in charge of setting schedules is an example of a person who is authoritative.

Authoritative describes someone who knows a great deal about a certain subject and who is generally considered an expert. (adjective)

A book written by an expert on certain subject matter is an example of an authoritative book.

See authoritative in Webster's New World College Dictionary

See authoritative in American Heritage Dictionary 4

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