Definition - Noun 1: the person or thing through which power is exerted or an end is achieved <death by criminal ~ W. R. LaFave and A. W. Scott, Jr.> 2 a: a consensual fiduciary relationship in which one party acts on behalf of and under the control of another in dealing with third parties also : the power of one in such a relationship to act on behalf of another A principal is bound by and liable for acts of his or her agent that are within the scope of the agency.