The currency of the United States is formally referred to as the U.S. Dollar, but forex traders call the world's primary reserve currency by a variety of colorful names.

These unique monikers go considerably beyond using just "the Dollar" since that can be confused with the currencies of other countries like Canada, Australia and New Zealand who also refer to their currencies as dollars.

The term Greenback has achieved particular popularity among forex market commentators when referring to the U.S. currency. This term has a long history, reaching all the way back to the Civil War era in America in the late 1800's when then-President Abraham Lincoln printed U.S. Demand Notes to finance the war.

These notes were printed in black ink on the front and green ink on the back, unlike most other currency notes of that era which had blank backs, thereby giving rise to the name.

To this day, U.S. Dollars are printed with green ink on their backs, keeping consistent with this traditional name for the currency.

Another popular forex market nickname for the U.S. Dollar, especially among forex traders themselves, is the "buck". In fact, it would not be at all uncommon for an interbank forex trader to ask a broker or another professional counterparty for a price in "ten million bucks" or simply "ten bucks", meaning that they need a price in ten million dollars.