1. To do something; to exert energy or some force; to be employed or operative: Henry acted promptly in helping the people in the auto accident.
2. To reach, to make, or to issue a decision on some matter: The members congregated to act on the bill.
3. To operate or function in a particular way; to perform specific duties or functions: Shirley was acting as a child's substitute mother until her parents were able to return home.