All legal tender recognised by governments and banks as a means of payment. Fiat currencies are for instance the US Dollar, Euro, Pound Sterling, Yen and so on. The value of a fiat currency such as the Dollar is attributed by a central bank, in this case the Federal Reserve. They control the distribution and assesment of the fiat currency. This is because fiat is not indexed to the price of a natural resource such as gold or silver. So, just like cryptocurrencies, the Dollar or the Euro (in their physical representation i.e. banknotes) don’t actually correspond to a tangible good like gold or silver. They are simply accepted as a form of legal payment in a given country.