Definition of "Agent" :

noun: the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause

noun: an active and efficient cause; capable of producing a certain effect

"Their research uncovered new disease agents."

noun: a businessman who buys or sells for another in exchange for a commission

noun: a representative who acts on behalf of other persons or organizations

noun: any agent or representative of a federal agency or bureau

noun: a substance that exerts some force or effect