A Power of Attorney (POA) 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 act is the principal, granter or donor (of the power), and the one authorized to act is the agent, or attorney in fact or in many common law jurisdictions, simply the attorney.
As an 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 usually separate from the power of attorney itself, so if the 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.