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Money from WikiPedia:

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 is an abstraction, idea or concept, token instances of which are the physical bills or coins which are carried and traded.

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". By law, the refusal of a legal tender (offering) extinguishes the debt in the same way acceptance does. Some bullion coins such as the Australian Gold Nugget and American Eagle are legal tender, however, they trade based on the market price of the metal content as a commodity, rather than their legal tender face value (which is usually only a small fraction of their bullion value).

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