Currency is the medium of exchange for goods and services. Tokens used as a mode of payment including metal coins and paper bank notes, currency also includes checks drawn on bank accounts, money orders, traveler checks, and will soon include electronic money and digital cash. The value of any currency fluctuates constantly in relation to other currencies. The currency exchange market exists as a means of profiting from those fluctuations. A key characteristic of modern money is that it is uniformly worthless in itself. That is, bills are pieces of paper rather than coins made of gold, silver, or bronze.

Doug Casey: A currency is a government substitute for money