Gresham's law

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

• noun The theory holding that if two kinds of money in circulation have the same denominational value but different intrinsic values, the money with higher intrinsic value will be hoarded and eventually driven out of circulation by the money with lesser intrinsic value.

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

• noun (economics) the principle that when two kinds of money having the same denominational value are in circulation the intrinsically more valuable money will be hoarded and the money of lower intrinsic value will circulate more freely until the intrinsically more valuable money is driven out of circulation; bad money drives out good; credited to Sir Thomas Gresham