It was interesting to note that the nickname Greenback is attributed to Abraham Lincoln. It is recorded that the name Greenback was used ever since Lincoln created Demand Notes to finance the cost of war efforts during the American Civil War. The original note was printed in the colors black and green on the back of the note.
Though it is a nickname of the 19th century U.S. Dollar, it is still popularly used to refer to the currency of the United States. Yet another story is that greenbacks refer to the dead Presidents of the United States because late U.S. Presidents are printed on some of the Dollar bills. Look at the $100 bills in the above picture showing the photo of George Washington, the first President of the United States, who was in office from April 30, 1789 to March 4, 1797.
The name Dollar is used for the currency notes of some other countries such as Canada, but the term Greenback specifically refers to the U.S. currency only.