Real Dictionary

Gresham's law in American English
the theory that when two or more kinds of money of equal denomination but unequal intrinsic value are in circulation, the one of greater value will tend to be hoarded or exported; popularly, the principle that bad money will drive good money out of circul
noun: the tendency of the inferior of two forms of currency to circulate more freely than, or to the exclusion of, the superior, because of the hoarding of the latter

Gresham's law in British English
noun: the economic hypothesis that bad money drives good money out of circulation; the superior currency will tend to be hoarded and the inferior will thus dominate the circulation