The Collaborative International Dictionary of English v.0.48:

Countermand \Coun"ter*mand\ (koun"t[~e]r*m[.a]nd), n.
   A contrary order; revocation of a former order or command.
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         Have you no countermand for Claudio yet,
         But he must die to-morrow?               --Shak.
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The Collaborative International Dictionary of English v.0.48:

Countermand \Coun`ter*mand"\ (koun`t[~e]r*m[.a]nd"), v. t. [imp.
   & p. p. Countermanded; p. pr. & vb. n. Countermanding.]
   [F. contremander; contre (L. contra) + mander to command, fr.
   L. mandare. Cf. Mandate.]
   1. To revoke (a former command); to cancel or rescind by
      giving an order contrary to one previously given; as, to
      countermand an order for goods.
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   2. To prohibit; to forbid. [Obs.]
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            Avicen countermands letting blood in choleric
            bodies.                               --Harvey.
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   3. To oppose; to revoke the command of.
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            For us to alter anything, is to lift ourselves
            against God; and, as it were, to countermand him.
                                                  --Hooker.
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WordNet (r) 3.0 (2006):

countermand
    n 1: a contrary command cancelling or reversing a previous
         command
    v 1: cancel officially; "He revoked the ban on smoking"; "lift
         an embargo"; "vacate a death sentence" [syn: revoke,
         annul, lift, countermand, reverse, repeal,
         overturn, rescind, vacate]