In economics, money is any financial instrument that can fulfill the functions of money (detailed above). These financial instruments together are collectively referred to as the money supply of an economy. In other words, the money supply is the number of financial instruments within a specific economy available for purchasing goods or services. Since the money supply consists of various financial instruments (usually currency, demand deposits, and various other types of deposits), the amount of money in an economy is measured by adding together these financial instruments creating a monetary aggregate.