agent

(noun) an active and efficient cause; capable of producing a certain effect. Example: "Their research uncovered new disease agents."

(noun) a representative who acts on behalf of other persons or organizations.

(noun) a substance that exerts some force or effect.

(noun) a businessman who buys or sells for another in exchange for a commission. Synonyms: factor, broker.

(noun) any agent or representative of a federal agency or bureau. Synonyms: federal agent.

(noun) the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause. Synonyms: agentive role.