【Agent】 , 【Factor】 , 【Attorney】 , 【Deputy】 and 【Proxy】 all agree in meaning one who performs the duties of or transacts business for another, but differ in specific application.

【Agent】 is very general and may be used to express this idea in any context where a specific term is not required; distinctively, however, it often implies the activity of a go-between.

• ambassadors, ministers, emissaries, nuncios are diplomatic agents of their governments or sovereigns
• the heads of departments are the political or confidential agents of the executive
  —John Marshall
• let every eye negotiate for itself and trust no 【agent】
  —Shak.

【Factor】 was once a near equivalent of 【agent】 but is now chiefly employed as a designation for a commercial 【agent】 buying and selling goods for others on commission.

• wool 【factor】
• flour 【factor】

It is also used specifically to name the official in charge of a trading post of the Hudson’s Bay Company. 【Attorney】 , now chiefly used as a designation for a professional legal 【agent】 (see LAWYER), once was applied to one who performed the personal offices of another who was absent, incapacitated, or unqualified for the work.

• I will attend my husband, be his nurse . . . for it is my office, and will have no 【attorney】 but myself
  — Shak.

This sense still survives but in a narrower application to a person legally delegated to transact certain specified business for another who is absent or otherwise disqualified. Such a person is often called (in distinction from an 【attorney】 -at-law) a "private 【attorney】 " or an " 【attorney】 -in-fact," and the power delegated him is called "power of 【attorney】 ."

【Deputy】 always implies possession through delegation of some or all of the powers of a superior (as a sovereign or a governmental or business executive).

Almost always, also, it connotes responsibility to the person whose powers are deputed, rather than to the organization from which these powers ultimately derive.

• the governor-general of Canada may appoint deputies to exercise his powers or functions locally or temporarily
• a vicar-general is a 【deputy】 of a bishop

【Proxy】 implies a substitution of persons when a promise or pledge is solemnly made or a vote, as at a stockholders’ meeting, is to be cast.

In a marriage service a 【proxy】 for the bride or groom or in the baptismal service a 【proxy】 for a godparent merely utters the promises in the name of the absent person, the latter assuming the obligation of fulfilling them.