Money is anything that is used in the conduct of trade and the repayment of debt.
In barter economies goods exchange for goods, in monetary economies goods exchange for money. A direct exchange of goods between two individuals in a monetary economy results in an incomplete transaction because goods must be first sold for money and then money is used to buy other goods. Satisfying double coincidence in economies with many goods becomes almost impossible. This good or money is used to conduct trade as long as people are confident that others will accept it in trade. That is, it must be able to maintain its purchasing power.