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.
   [1913 Webster]

         Have you no countermand for Claudio yet,
         But he must die to-morrow?               --Shak.
   [1913 Webster]

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.
      [1913 Webster]

   2. To prohibit; to forbid. [Obs.]
      [1913 Webster]

            Avicen countermands letting blood in choleric
            bodies.                               --Harvey.
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   3. To oppose; to revoke the command of.
      [1913 Webster]

            For us to alter anything, is to lift ourselves
            against God; and, as it were, to countermand him.
                                                  --Hooker.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "countermand":
   abolish, abolishment, abolition, abrogate, abrogation, annul,
   annulment, baffle, balk, blast, brave, cancel, canceling,
   cancellation, cassation, challenge, checkmate, circumvent,
   confound, confront, contravene, counter, counteract, counterorder,
   counterwork, cross, dash, defeasance, defeat, defy, destroy,
   disannul, discomfit, disconcert, discountenance, dish, disrupt,
   do away with, elude, flummox, foil, frustrate, invalidate,
   invalidation, knock the chocks, make void, nonplus, nullification,
   nullify, override, overrule, perplex, recall, recant, recantation,
   renege, repeal, rescind, rescinding, rescindment, rescission,
   retract, retraction, reversal, reverse, revocation, revoke,
   revokement, ruin, sabotage, scotch, set aside, setting aside,
   spike, spoil, stonewall, stump, suspend, suspension, thwart, upset,
   vacate, vacation, vacatur, void, voidance, voiding, waive, waiver,
   waiving, withdraw, withdrawal, write off, write-off



Bouvier's Law Dictionary, Revised 6th Ed (1856):

COUNTERMAND. This word signifies a. change or recall of orders previously
given.
     2. It may be express or implied. Express, when contrary orders are
given and a revocation. of the former order is made. Implied, when a new
order is given which is inconsistent with the former order: as, if a man
should order a merchant to ship him in a particular vessel certain goods
which belonged to him, and then, before the goods were shipped, he directed
him to ship them in another vessel; this would be a countermand of the first
order.
     3. While the first command is unrecalled, the person who gave it would
be liable to all the consequences in case he should be obeyed; but if, for
example, a man should command another to commit a crime and, before its
perpetration, he should repent and countermand it, he would not be liable
for the consequences if the crime should afterwards be committed.
     4. When a command or order has been given, and property delivered, by
which a right vests in a third person, the party giving the order cannot
countermand it; for example, if a debtor should deliver to A a sum of money
to be paid to B, his creditor, B has a vested right in the money, and unless
he abandon that right, and refuse to take the money, the debtor cannot
recover it from A. 1 Roll. Ab. 32, pl. 13; Yelv. 164 Sty. 296. See 3 Co. 26
b.; 2 Vent. 298 10 Mod. 432; Vin. Ab. Countermand, A 1; Vin. Ab. Bailment,
D; 9 East, 49; Roll. Ab. 606; Bac. Ab. Bailment, D; Com. Dig. Attorney, B 9,
c. 8; Dane's Ab. h.t.; and Command.