The Collaborative International Dictionary of English v.0.48:

Counterpoise \Coun"ter*poise`\ (koun"t?r-poiz`; 277), v. t.
   [imp. & p. p. Counterpoised (-poizd`); p. pr. & vb. n.
   Counterpoising.] [OE. countrepesen, counterpeisen, F.
   contrepeser. See Counter, adv., and Poise, v. t. ]
   1. To act against with equal weight; to equal in weight; to
      balance the weight of; to counterbalance.
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            Weights, counterpoising one another.  --Sir K.
                                                  Digby.
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   2. To act against with equal power; to balance.
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            So many freeholders of English will be able to beard
            and counterpoise the rest.            --Spenser.
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The Collaborative International Dictionary of English v.0.48:

Counterpoise \Coun"ter*poise`\ (koun"t?r-poiz`), n. [OE.
   countrepese, OF. contrepois, F. contrepods. See Counter,
   adv., and Poise, n.]
   1. A weight sufficient to balance another, as in the opposite
      scale of a balance; an equal weight.
      [1913 Webster]

            Fastening that to our exact balance, we put a
            metalline counterpoise into the opposite scale.
                                                  --Boyle.
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   2. An equal power or force acting in opposition; a force
      sufficient to balance another force.
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            The second nobles are a counterpoise to the higher
            nobility, that they grow not too potent. --Bacon.
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   3. The relation of two weights or forces which balance each
      other; equilibrium; equiponderance.
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            The pendulous round eart, with balanced air,
            In counterpoise.                      --Milton.
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WordNet (r) 3.0 (2006):

counterpoise
    n 1: a weight that balances another weight [syn:
         counterweight, counterbalance, counterpoise,
         balance, equalizer, equaliser]
    v 1: constitute a counterweight or counterbalance to [syn:
         counterweight, counterpoise, counterpose]