Fiat money is money that derives its value from government regulation or law. The term derives from the Latin fiat, meaning “let it be done” or “it shall be [money]”, as such money is established by government decree. Where fiat money is used as currency, the term fiat currency is used.
Fiat Money Explained
By World War I most nations had a legalized government monopoly on bank notes and the legal tender status thereof. In theory, governments still promised to redeem notes in specie on demand. However, the costs of the war and the massive expansion afterward made governments suspend redemption in specie. Since there was no direct penalty for doing so, governments were not immediately responsible for the economic consequences of printing more money, which led to hyperinflation – for example in Weimar Germany.
Attempts were made to reassert currency stability by anchoring it to wholesale gold bullion rather than making it payable in specie. This money combined pure fiat currency, in that the currency was limited to central bank notes and token coins that were current only by government fiat, with a form of convertibility, via gold bullion exchange, or via exchange into US dollars which were convertible into gold bullion, under the 1945 Bretton Woods system.
This explains what fiat money is, how we produce and how we use it. A good read with more examples from US history can be found here.