attorney-in-fact
A person authorized to act on another's behalf through a written power of attorney. Used in legal and business contexts.
The attorney-in-fact signed the contract.
He proceeded with the real estate transaction as an attorney-in-fact.
Compound of 委任状 (power of attorney) + による (by means of) + 代理人 (agent). The phrase is a direct legal term without a deeper historical derivation.