A power of attorney 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, sometimes against the wishes of the other. The person authorizing the other to act is the principal, grantor, or donor (of the power). The one authorized to act is the agent or, in some common law jurisdictions, the attorney-in-fact.
Formerly, the term "power" referred to an instrument signed under seal while a "letter" was an instrument under hand, meaning that it was simply signed by the parties, but today a power of attorney need not be signed under seal. Some jurisdictions require that powers of attorney be notarized or witnessed, but others will enforce a power of attorney as long as it is signed by the grantor.