Money is anything that is generally accepted as payment for goods and services and repayment of debts. The main functions of money are distinguished as: a medium of exchange, a unit of account, a store of value, and occasionally, a standard of deferred payment. Money originated as commodity money, but nearly all contemporary money systems at the national level are fiat money systems. Fiat money is without value as a physical commodity, and derives its value by being declared by a government to be legal tender; that is, it must be accepted as a form of payment within the national boundaries of the country, for "all debts, public and private".
The money supply of a country is usually held to consist of currency and demand deposits or 'bank money'. These demand deposits usually account for a much larger part of the money supply than currency.
There have been many historical disputes regarding the combination of money's functions, some arguing that they need more separation and that a single unit is insufficient to deal with them all. One of these arguments is that the role of money as a medium of exchange is in conflict with its role as a store of value: its role as a store of value requires holding it without spending, whereas its role as a medium of exchange requires it to circulate. Others argue that storing of value is just deferral of the exchange, but does not diminish the fact that money is a medium of exchange that can be transported both across space and time. The term 'financial capital' is a more general and inclusive term for all liquid instruments, whether or not they are a uniformly recognized tender.
Money as medium of exchange
When money is used to intermediate the exchange of goods and services, it is performing a function as a medium of exchange. It thereby avoids the inefficiencies of a barter system, such as the 'double coincidence of wants' problem.
A medium of exchange is an intermediary used in trade to avoid the inconveniences of a pure barter system.
By contrast, as William Stanley Jevons argued, in a barter system there must be a coincidence of wants before two people can trade – one must want exactly what the other has to offer, when and where it is offered, so that the exchange can occur. A medium of exchange permits the value of goods to be assessed and rendered in terms of the intermediary, most often, a form of money widely accepted to buy any other good.
Money as store of value
To act as a store of value, a commodity, a form of money, or financial capital must be able to be reliably saved, stored, and retrieved — and be predictably useful when it is so retrieved. Fiat currency like paper or electronic money no longer backed by gold in most countries is not considered by some economists to be a store of value.