The Federal Reserve Note is the official currency of the United States, issued by the Federal Reserve System. However, there are several synonyms for the Federal Reserve Note that are commonly used in the country. One synonym is "greenback," which refers to the US dollar notes that have green ink. Another term is "buck," which is a slang word for the US dollar - a shortened form of "buckskin," which was what people used to trade with before the invention of cash. "Dollar bill" is also a synonym for the Federal Reserve Note. "Green," "cash," and "money" are other informal terms used to describe the Federal Reserve Note.