agent
Cross references: 1. mean agent \'a--j*nt\ n [ME, fr. ML agent-, agens, fr. L, prp. of agere to drive, lea]d, act, do; akin to ON aka to travel in a vehicle, Gk agein to drive, lead 1a: something that produces or is capable of producing an effect : an act ive or efficient cause 1b: a chemically, physically, or biologically active principle 2: one that acts or exerts power 3: a person responsible for his acts 4: MEANS Instrument 5: one who acts for or in the place of another by authority from him
Webster's English Dictionary