medium of exchange

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun anything that is generally accepted as a standard of value and a measure of wealth in a particular country or region