What is Inherent Authority?

Legal Definition
An agent's power to act on behalf of a principal, even though not expressly or impliedly granted. This power arises only if required for the agent to exercise some actual authority granted by the same principal. Thus, if a principal grants power to do X, and X requires doing Y, then the agent acquires inherent authority to do Y on behalf of the principal.

See Express authority, Actual authority, Implied authority, and Apparent authority.