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

AGENT is a 5 letter word that starts with A.

1. \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.
   

2. \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.
   

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.