agent

noun

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
5. any agent or representative of a federal agency or bureau
6. the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause