noun
One who acts for another
An instrumentality
an agent is, generally, a person or thing that acts or is capable of acting, or, in this comparison, one who is empowered to act for another a literary agent; factor, a less common term, now usually denotes an agent for the sale of goods; a deputy is a public official to whom certain authority has been delegated by superiors; proxy implies the delegation of power to substitute for another in some formal or ceremonial act some stockholders vote by proxy
See agent in Webster's New World Roget's A-Z Thesaurus II
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