"agent" in the noun sense
an active and efficient cause capable of producing a certain effect
"their research uncovered new disease agents"
a representative who acts on behalf of other persons or organizations
a substance that exerts some force or effect
4. agent, factor, broker
a businessman who buys or sells for another in exchange for a commission
5. agent, federal agent
any agent or representative of a federal agency or bureau
6. agentive role, agent
the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause
Source: WordNet® (An amazing lexical database of English)
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