The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Agent \A"gent\, n.
   1. One who exerts power, or has the power to act; an actor.
      [1913 Webster]

            Heaven made us agents, free to good or ill.
                                                  --Dryden.
      [1913 Webster]

   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.
      [1913 Webster]

   3. An active power or cause; that which has the power to
      produce an effect, such as a physical, chemical, or
      medicinal agent; as, heat is a powerful agent.
      [1913 Webster]

   4. (Biochem., Med.) a chemical substance having biological
      effects; a drug.
      [PJC]

WordNet (r) 3.0 (2006):

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

The Free On-line Dictionary of Computing (19 January 2023):

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)