The person to whom authority is given under a Power of Attorney. An attorney in fact is sometimes referred to as a "proxy" under a Health Care Power of Attorney.

A person to whom the authority of another, 'the constituent', is by him lawfully delegated. This term is employed to designate persons who act under a special agency, or a special letter of attorney, so that they are appointed in factum, for the deed, or special act to be performed; but in a more extended sense it includes all other agents employed in any business, or to do any act or acts in pais for another.