Which currency is known as the 'greenback'?

The currency known as the “greenback” is the United States dollar.

The nickname refers to the green ink used on the reverse side of many United States paper notes. Green became especially associated with U.S. currency after the federal government issued Demand Notes during the American Civil War in 1861. The color helped distinguish the notes and was also difficult for contemporary color-copying methods to reproduce accurately.

“Greenback” originally referred particularly to the government-issued paper money known as United States Notes, whose backs were printed in green. In everyday modern usage, however, the term is a familiar nickname for U.S. dollar banknotes and, more broadly, for the dollar itself.

The expression is not the dollar’s formal name, and it does not mean every dollar must be physically green. U.S. coins are metallic, and modern notes use many security features, including watermarks, color-shifting ink, and embedded security threads. Other dollar nicknames include “buck,” while “dollar” remains the official everyday currency name.