dollar vs greenback

dollar

noun
  • Imported from the United States, and paid for in U.S. dollars. (Note: distinguish "dollar wheat", North American farmers' slogan, meaning a market price of one dollar per bushel.) 

  • A quarter of a pound or one crown, historically minted as a coin of approximately the same size and composition as a then-contemporary dollar coin of the United States, and worth slightly more. 

  • Official designation for currency in some parts of the world, including Canada, the United States, Australia, New Zealand, Hong Kong, and elsewhere. Its symbol is $. 

  • Money generally. 

  • A unit of reactivity equal to the interval between delayed criticality and prompt criticality. 

greenback

noun
  • The United States dollar. 

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

  • 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 dollar and greenback occurred in a corpus of books? (source: Google Ngram Viewer )