A power of attorney (POA) or letter of attorney is a written authorization to represent or act on another's behalf in private affairs (which may be financial or regarding health and welfare), business, or some other legal matter. The person authorizing the other to act is the principal, grantor, or donor (of the power). The one authorized to act is the agent, attorney, 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 does not need to 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.