someone who is sent to another country as an official representative.——朗文当代大辞典

A public minister sent by one sovereign or government to another for the transaction of diplomatic business. Now applied esp. to diplomatic ministers of the second rank (‘ministers plenipotentiary’) as distinguished from those of the highest rank (‘ambassadors’), and those of the third rank (‘chargés d'affaires’).——牛津当代大辞典  @en