medium of exchange

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Something, such as a precious metal, that is commonly used in a specific area or among a certain group of people as money.

Wiktionary

1. n. An intermediary used in trade to avoid the inconveniences of a pure barter system, such as money.

GNU Webster's 1913

1. n. that which is used for effecting an exchange of commodities -- money or current representatives of money.

WordNet 3.0

1. n. anything that is generally accepted as a standard of value and a measure of wealth in a particular country or region