A currency is a standardization of money in any form, in use or circulation as a medium of exchange, for example banknotes and coins. A more general definition is that a currency is a system of money in common use within a specific environment over time, especially for people in a nation state. Under this definition, the British Pound Sterling (£), euros (€), Japanese yen (¥), and U.S. dollars (US$) are examples of (government-issued) fiat 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 either chosen by users or decreed by governments, and each type has limited boundaries of acceptance; i.e., legal tender laws may require a particular unit of account for payments to government agencies.

Other definitions of the term "currency" appear in the respective synonymous articles: banknote, coin, and money. This article uses the definition which focuses on the currency systems of countries.

One can classify currencies into three monetary systems: fiat money, commodity money, and representative money, depending on what guarantees a currency's value (the economy at large vs. the government's physical metal reserves). Some currencies function as legal tender in certain jurisdictions, or for specific purposes, such as payment to a government (taxes), or government agencies (fees, fines). Others simply get traded for their economic value.

Digital currency has arisen with the popularity of computers and the Internet. Whether government-backed digital notes and coins (such as the digital renminbi in China, for example) will be successfully developed and utilized remains dubious. Decentralized digital currencies, such as cryptocurrencies, are different because they are not issued by a government monetary authority; specifically, bitcoin, the first cryptocurrency and leader in terms of market capitalization, has a fixed supply and is therefore ostensibly deflationary. Many warnings issued by various countries note the opportunities that cryptocurrencies create for illegal activities such as scams, ransomware, money laundering and terrorism. In 2014, the United States IRS issued a statement explaining that virtual currency is treated as property for Federal income-tax purposes, and it provide examples of how long-standing tax principles applicable to transactions involving property apply to virtual currency.