agent in WordNet English dictionary

1. the semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause
   agentive role  agent 
   
2. an active and efficient cause; capable of producing a certain effect
   "their research uncovered new disease agents"
   agent 
   
3. a businessman who buys or sells for another in exchange for a commission
   agent  factor  broker 
   
4. a representative who acts on behalf of other persons or organizations
   agent 
   
5. any agent or representative of a federal agency or bureau
   agent  federal agent 
   
6. a substance that exerts some force or effect
   agent