An agent authorized to act on behalf of another person, but not necessarily authorized to practice law, e.g. a person authorized to act by a power of attorney. An attorney in fact is a fiduciary. Also known as attorney-in-fact or private attorney.
For example, Person A might give a power of attorney to Person B that allows Person B to manage Person A's bank accounts. In this example, Person A is the principal, and Person B is the attorney in fact.
See, e.g. Sperry v. State of Fla. ex. rel Florida Bar, 373 U.S. 379 (1963).