Existing definitions of agency are inadequate. In this paper, I offer a new definition of agency: a relationship is one of agency where the agent has a power to exercise at least one of the principal's own powers. I call this a “proxy power” for short. This definition brings with it three advantages. First, focusing on the agent's proxy power highlights the distinctive idea at the core of agency: that people can act through others at law, thereby expanding their legal personality in space. Second, that distinctive idea then helps identify the three reasons justifying when an agent has a proxy power: the principal's unilateral manifestation of will, the necessity of protecting the principal's interests, and where the principal is an artificial person who can only unilaterally manifest his will through an agent. Third, the definition vindicates prevailing intuitions about agency, and gives us a more transparent way to decide whether or one's invocation of “agency” is illegitimate or legitimate.