The Collaborative International Dictionary of English v.0.48:

Countercheck \Coun`ter*check"\ (koun`t?r-ch?k"), v. t. [imp. &
   p. p. Counterchecked (-ch?ckt"); p. pr. & vb. n.
   Counterchecking.]
   To oppose or check by some obstacle; to check by a return
   check.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Countercheck \Coun"ter*check`\ (koun"t?r-ch?k`), n.
   1. A check; a stop; a rebuke, or censure to check a reprover.
      [1913 Webster]

   2. Any force or device designed to restrain another
      restraining force; a check upon a check.
      [1913 Webster]

            The system of checks and counterchecks. --J. H.
                                                  Newton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

countercheck
    n 1: a check that restrains another check
    2: something that checks the correctness of a previous check
       [syn: countercheck, double check]
    v 1: oppose or check by a counteraction [syn: countercheck,
         counteract]
    2: check a second time