The term "Greenback" refers to paper currency issued by the United States during the Civil War, specifically starting in 1861. These notes were called "Greenbacks" because of their green ink on the back. They were initially used to help finance the war and were not backed by gold or silver, leading to fluctuations in their value. Over time, Greenbacks became a significant part of the U.S. monetary system. In 1863, the National Banking Act established a system of national banks that could issue currency backed by U.S. government bonds, further stabilizing the value of Greenbacks and paving the way for modern currency.