A power of attorney or letter of attorney in common law systems or mandate in civil law systems is an authorization to act on someone else's behalf in a legal or business matter. The person authorizing the other to act is the "principal" or "grantor", and the one authorized to act is the "agent" or "attorney-in-fact". The attorney-in-fact acts "in the principal's name," signing the principal's name to documents and filing suit with the principal's name as plaintiff, for example.

 As one kind of agent, an attorney-in-fact is a fiduciary for the principal, so the law requires an attorney-in-fact to be completely honest with and loyal to the principal in their dealings with each other. If the attorney-in-fact is being paid to act for the principal, the contract is a separate matter from the power of attorney itself, so if that contract is in writing, it is a separate document, kept private between them, whereas the power of attorney is intended to be shown to various other people.

The power of attorney may be oral, such as asking someone else to sign your name on a cheque because your arm is broken, or may be in writing. Many institutions, such as hospitals, banks, and the I.R.S., require a power of attorney to be in writing before they will honor it, and they usually want to keep an original for their records.