Русско-англо-русский юридический словарь

A medium of exchange is an intermediary used in trade to avoid the inconveniences of a pure barter system. 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. (Wikipedia)

After all, money is anything that can perform three basic functions: a medium of exchange to be used in the purchase and sale of goods and services; a unit of account to measure the value of goods and services; and a store of value that can be saved and spent at a future point in time. (BBC News)