An attorney-in-fact, also called an agent, is a person who is authorized to act on behalf of another person, known as the principal, typically to perform business or other official transactions.  
The principal usually designates someone as their attorney-in-fact by assigning them power of attorney, although a court may choose to assign it if the person being represented is incapacitated. The rules regulating power of attorney vary from state to state. 
An attorney-in-fact is not necessarily a lawyer. Power of attorney may also be granted to more than one person.