High Quality Content by WIKIPEDIA articles! A power of attorney (POA) or letter of attorney is a written authorization to represent or act on another`s behalf in private affairs, business, or some other legal matter. The person authorizing the other to act is the principal, grantor, or donor (of the power), and the one authorized to act is the agent, donee, or attorney or, in some common law jurisdictions, the attorney-in-fact. Formerly, a power referred to an instrument under seal while a letter was an instrument under hand, but today both are under hand (i.e., signed by the donor), and therefore there is no difference between the two.