The informal name given to the US dollar because of its green banknotes is greenback.
The nickname originally referred to United States Demand Notes issued during the American Civil War. Authorized in 1861 and 1862, these notes used green ink prominently on their reverse side, helping distinguish them from other forms of paper money and inspiring the term “greenback.” They were not initially redeemable for a fixed quantity of gold or silver, making them an important early form of US government fiat paper currency.
Today, greenback is a familiar informal name for the US dollar, especially in financial journalism. It is not the dollar’s ISO code—that is USD—and it is not the same as “buck,” a broader colloquial term for a dollar. “Greenie” is not the standard nickname.
Modern Federal Reserve Notes remain predominantly green on their backs, although their security features, designs, and colors have changed over time. The original historical greenbacks and today’s dollar bills are therefore related by nickname, not identical in legal form.