Agent
 

agent
noun1.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 organizations3.a substance that exerts some force or effect4.a businessman who buys or sells for another in exchange for a commission5.any agent or representative of a federal agency or bureau6.the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause[syn: agentive role]