Source : Webster's Revised Unabridged Dictionary (1913)

Agent \A"gent\, a. [L. agens, agentis, p. pr. of agere to act;
   akin to Gr. ? to lead, Icel. aka to drive, Skr. aj. [root]2.]
   Acting; -- opposed to {patient}, or sustaining, action.
   [Archaic] ``The body agent.'' --Bacon.

Agent \A"gent\, n.
   1. One who exerts power, or has the power to act; an actor.

            Heaven made us agents, free to good or ill.
                                                  --Dryden.

   2. One who acts for, or in the place of, another, by
      authority from him; one intrusted with the business of
      another; a substitute; a deputy; a factor.

   3. An active power or cause; that which has the power to
      produce an effect; as, a physical, chemical, or medicinal
      agent; as, heat is a powerful agent.

Source : WordNet®

agent
     n 1: an active and efficient cause; capable of producing a
          certain effect; "their research uncovered new disease
          agents"
     2: a substance that exerts some force or effect
     3: a representative who acts on behalf of other persons or
        organizations
     4: a businessman who buys or sells for another in exchange for
        a commission [syn: {factor}, {broker}]
     5: any agent or representative of a federal agency or bureau
        [syn: {federal agent}]
     6: the semantic role of the animate entity that instigates or
        causes the hapening denoted by the verb in the clause
        [syn: {agentive role}]

Source : Free On-Line Dictionary of Computing

agent
     
         In the {client-server} model, the part of the
        system that performs information preparation and exchange on
        behalf of a {client} or {server}.  Especially in the phrase
        "intelligent agent" it implies some kind of automatic process
        which can communicate with other agents to perform some
        collective task on behalf of one or more humans.
     
        (1995-04-09)