mediator vs peacekeeper

mediator

noun
  • One who negotiates between parties seeking mutual agreement. 

  • A chemical substance transmitting information to a targeted cell. 

peacekeeper

noun
  • One that preserves the peace. 

  • An officer of the peace (police officer) in a force run by a Canadian aboriginal community. 

  • A member of a military force charged with peacekeeping duties in a troublespot. 

How often have the words mediator and peacekeeper occurred in a corpus of books? (source: Google Ngram Viewer )