Source : WordNet®

Gresham's Law
     n : (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