The Collaborative International Dictionary of English v.0.48:

Counterfoil \Coun"ter*foil`\ (-foil), n. [Counter- + foil a
   leaf.]
   [1913 Webster]
   1. That part of a tally, formerly in the exchequer, which was
      kept by an officer in that court, the other, called the
      stock, being delivered to the person who had lent the king
      money on the account; -- called also counterstock.
      [Eng.]
      [1913 Webster]

   2. The part of a writing (as the stub of a bank check) in
      which are noted the main particulars contained in the
      corresponding part, which has been issued.
      [1913 Webster]

WordNet (r) 3.0 (2006):

counterfoil
    n 1: the part of a check that is retained as a record [syn:
         stub, check stub, counterfoil]