Definition - Noun 1: someone or something that acts or exerts power : a moving force in achieving some result 2: a person guided or instigated by another in some action <where the heads of departments are the political...~s of the executive, merely to execute the will of the president Marbury v. Madison, 5 U.S. 137 (1803)> see also innocent agent 3 a: a person or entity (as an employee or independent contractor) authorized to act on behalf of and under the control of another in dealing with third parties see also agency fiduciary relationshipsubagent compare fiduciaryprincipalservant