On a deeper level, money fulfills two important requirements. First, money must be easily exchangeable for other assets. To meet this liquidity test, money must be readily acceptable in exchange for all other goods and services. Second, money must have substance, or in other words, it must be an asset so that in an exchange of money for a good or service, substance is exchanged for substance. Only in this way can the exchange be extinguished, which means that it ends at the moment of the transaction without any ongoing and lingering obligation.
Transactions are only extinguished when goods or services are exchanged for other assets.
National currencies are essentially not money. But today national currencies are no longer convertible into specie and have little, if any, precious metal foundation. For example, less than two cents of every US Dollar in circulation is backed by Gold. Fiat currencies are generally brought into circulation by government decree through legal tender laws. These laws are, in effect, a government sanctioned monopoly on the power to create and issue currency.