A power

Attorney At*tor"ney, n.; pl. {Attorneys}. [OE. aturneye, OF. atorn['e], p. p. of atorner: cf. LL. atturnatus, attornatus, fr. attornare. See {Attorn}.] 1. A substitute; a proxy; an agent. [Obs.] [1913 Webster]

And will have no attorney but myself. --Shak. [1913 Webster]

2. (Law) (a) One who is legally appointed by another to transact any business for him; an attorney in fact. (b) A legal agent qualified to act for suitors and defendants in legal proceedings; an attorney at law. [1913 Webster]

Note: An attorney is either public or private. A private attorney, or an attorney in fact, is a person appointed by another, by a letter or power of attorney, to transact any business for him out of court; but in a more extended sense, this class includes any agent employed in any business, or to do any act in pais, for another. A public attorney, or attorney at law, is a practitioner in a court of law, legally qualified to prosecute and defend actions in such court, on the retainer of clients. --Bouvier. -- The attorney at law answers to the procurator of the civilians, to the solicitor in chancery, and to the proctor in the ecclesiastical and admiralty courts, and all of these are comprehended under the more general term lawyer. In Great Britain and in some states of the United States, attorneys are distinguished from counselors in that the business of the former is to carry on the practical and formal parts of the suit. In many states of the United States however, no such distinction exists. In England, since 1873, attorneys at law are by statute called solicitors. [1913 Webster]

{A power}, {letter}, or {warrant}, {of attorney}, a written authority from one person empowering another to transact business for him. [1913 Webster]