1. an active and efficient cause; capable of producing a certain effect;"their research uncovered new disease agents"
2. a substance that exerts some force or effect
3. a representative who acts on behalf of other persons or organizations
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 hapening denoted by the verb in the clause