intermediary vs intervener

intermediary

noun
  • An agent acting as a mediator between sides to facilitate agreement or cooperation. 

  • One or several stages of an event which occurs after the start and before the end. 

  • A person or organisation in an intermediate position (in a transaction, agreement, supply chain etc.) 

adj
  • Intermediate. 

intervener

noun
  • One who intervenes. 

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