A currency in the most specific sense is money in any form when in use or circulation as a medium of exchange, especially circulating banknotes and coins.
A more general definition is that a currency is a system of money in common use, especially for people in a nation. Under this definition, U.S. dollars, euros, Indian rupee, Japanese yen, and pounds sterling are examples of currencies. Currencies may act as stores of value and be traded between nations in foreign exchange markets, which determine the relative values of the different currencies. Currencies in this sense are defined by governments, and each type has limited boundaries of acceptance.
Other definitions of the term "currency" appear in the respective synonymous articles: banknote, coin, and money.
One can classify currencies into three monetary systems: fiat money, commodity money, and representative money, depending on what guarantees a currency's value. Some currencies function as legal tender in certain political jurisdictions. Others simply get traded for their economic value.
Digital currency has arisen with the popularity of computers and the Internet.