A currency (from Middle English curraunt, meaning in circulation) in the most specific use of the word refers to money in any form when in actual use or circulation, as a medium of exchange, especially circulating paper money. This use is synonymous with banknotes, or (sometimes) with banknotes plus coins, meaning the physical tokens used for money by a government.
A much more general use of the word currency is anything that is used in any circumstances, as a medium of exchange. In this use, "currency" is a synonym for the concept of money.
A definition of intermediate generality is that a currency is a system of money (monetary units) in common use, especially in a nation. Under this definition, British pounds, U.S. dollars, and European euros are different types of currency, or currencies. Currencies in this definition need not be physical objects, but as stores of value are subject to trading between nations in foreign exchange markets, which determine the relative values of the different currencies. Currencies in the sense used by foreign exchange markets, are defined by governments, and each type has limited boundaries of acceptance.
The former definitions of the term "currency" are discussed in their respective synonymous articles banknote, coin, and money. The latter definition, pertaining to the currency systems of nations, is the topic of this article.