"Greenbacks" refer to paper currency issued by the United States during the Civil War. The term originated from the green ink used on the back of these notes, which were initially issued to help finance the war. Unlike gold or silver coins, greenbacks were not backed by physical commodities, leading to fluctuations in their value. Over time, greenbacks became a standard form of currency in the U.S. economy. In 1863, the National Banking Act established a system of national banks that could issue their own notes, further integrating greenbacks into everyday transactions. Eventually, the U.S. moved towards a more stable currency system, but the legacy of greenbacks remains significant in American financial history.