banknote vs greenback

banknote

noun
  • A piece of paper currency. 

  • A promissory note issued by a bank, payable at a given time to a specific beneficiary. 

greenback

noun
  • A unit of American currency issued during the Civil War by the Treasury Department. 

  • The United States dollar. 

  • A wave that has not yet begun to break. 

  • Any bill that is legal tender in the US (originally printed with green and black ink) issued by the Federal Reserve. 

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