From Traderpedia

A unit of exchange which facilitates the transfer of goods and services.

A currency zone is a country or region (e.g. the Eurozone) where a particular currency is dominant. Trade between currency zones is made possible through exchange rates, the prices at which currencies can be exchanged for each other. Currencies are often referred to as either floating (those freely traded) or fixed (pegged to some specific base), though some can be considered a combination of the two.

See also Foreign Exchange.