Definitions of agent:

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

  Example: "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
• noun:   the semantic role of the animate entity that instigates or causes the hapening denoted by the verb in the clause
• name:  A surname (very rare: popularity rank in the U.S.: #32500)