Any form of government-issued money that is used as legal tender by the population and government of a certain nation or region is known as a FIAT currency.
Today, FIAT currencies act as a government-regulated instrument and serve as a store of value, unit of account and medium of exchange. FIAT currencies are backed by the government that issued them, rather than a physical commodity (such as silver or gold).
The vast majority of current paper currencies, such as the US dollar, the Japanese yen, and the Euro, are FIAT currencies.
See: Blockchain, Cryptocurrency.