Currency in circulation refers to the paper money and coins that are circulating within an economy and are counted as part of the total money supply, which also includes demand deposits in banks.
Federal Reserve Notes are paper currency issued by the Federal Reserve System (FED) and circulated by the Federal Reserve Banks. They serve as liabilities of the Federal Reserve Banks and constitute obligations of the U.S. government.
A greenback is specifically a term for U.S. paper currency, named for the green ink used on the reverse side. It can also broadly refer to any paper money not backed by physical commodities like gold or silver.
Certificates issued by government (and, at times, private entities) that are generally accepted within the economy as reliable media of exchange.
