A power of attorney (POA) or letter of attorney is a written authorization to represent or act on another's behalf in private affairs, business, or some other legal matter, sometimes against the wishes of the other. The person authorizing the other to act is the principal, grantor, or donor (of the power). The one authorized to act is the agent or, in some common law jurisdictions, the attorney-in-fact (attorney for short). Formerly, a power referred to an instrument under seal while a letter was an instrument under hand, but today both are signed by the grantor, and therefore there is no difference between the two.
The term attorney-in-fact is used in several countries in place of the term agent and should be distinguished from the term attorney-at-law.
In the United States, an attorney-at-law is a lawyer — someone licensed to practice law in a particular jurisdiction.
The Uniform Power of Attorney Act employs the term agent. 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 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.
In the context of the unincorporated reciprocal inter-insurance exchange (URIE) the attorney-in-fact is a stakeholder/trustee who takes custody of the subscriber funds placed on deposit with him, and then uses those funds to pay insurance claims. When all the claims are paid, the attorney-in-fact then returns the leftover funds to the subscribers.
The person who creates a power of attorney, known as the grantor, can only do so when he/she has the requisite mental capacity. Suppose the grantor loses capacity to grant permission after the power of attorney has been created (for example, from Alzheimer's disease or a head injury in a car crash); then the power will probably no longer be effective. In some powers of attorney the grantor states that he/she wishes the document to remain in effect even after he/she becomes incapacitated. This type of power is commonly referred to as a durable power of attorney. If someone is already incapacitated, it is not possible for that person to execute a valid power. If a person does not have the capacity to execute a power of attorney (and does not already have a durable power in place), often the only way for another party to act on their behalf is to have a court impose a conservatorship or a guardianship.