The Collaborative International Dictionary of English v.0.48:

dollar bill \dollar bill\
   A paper note printed by the Treasury, or by one of the
   Federal Reserve Banks under authority of the treasury, having
   the value of one dollar. Also five dollar bill, ten dollar
   bill, etc.: notes with the value of five, ten, etc. dollars.
   Prior to 1964 such notes could be redeemed for the equivalent
   dollar value in silver coins, but in that year the backing of
   the currency with silver was discontinued. Such notes not
   convertible into precious metals at a fixed rate are called
   "fiat money", receiving their value solely from the good
   faith of the issuing government and the confidence of those
   using the currency.
   [PJC]

WordNet (r) 3.0 (2006):

dollar bill
    n 1: a piece of paper money worth one dollar [syn: dollar,
         dollar bill, one dollar bill, buck, clam]