convention vs rap session

convention

noun
  • A formal agreement, contract, rule, or pact. 

  • A treaty or supplement to such. 

  • A meeting or gathering. 

  • A practice or procedure widely observed in a group, especially to facilitate social interaction; a custom. 

  • The convening of a formal meeting. 

  • A formal deliberative assembly of mandated delegates. 

rap session

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