greenback vs paper money

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. 

paper money

noun
  • Cash in the form of banknotes. 

  • A type of joss paper resembling banknotes burnt to venerate the deceased, ancestor spirits, deities, etc. 

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