The Collaborative International Dictionary of English v.0.48:

Countervail \Coun`ter*vail"\ (koun`t?r-v?l"), v. t. [imp. & p.
   p. Countervailed (-v?ld); p. pr. & vb. n.
   Countervailing.] [OF. contrevaloir; contre (L. contra) +
   valoir to avail, fr. L. valere to be strong, avail. See
   Vallant.]
   To act against with equal force, power, or effect; to thwart
   or overcome by such action; to furnish an equivalent to or
   for; to counterbalance; to compensate.
   [1913 Webster]

         Upon balancing the account, the profit at last will
         hardly countervail the inconveniences that go allong
         with it.                                 --L'Estrange.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Countervail \Coun"ter*vail`\ (koun"t?r-v?l`), n.
   Power or value sufficient to obviate any effect; equal
   weight, strength, or value; equivalent; compensation;
   requital. [Obs.]
   [1913 Webster]

         Surely, the present pleasure of a sinful act is a poor
         countervail for the bitterness of the review. --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

countervail
    v 1: compensate for or counterbalance; "offset deposits and
         withdrawals" [syn: offset, countervail]
    2: oppose and mitigate the effects of by contrary actions; "This
       will counteract the foolish actions of my colleagues" [syn:
       counteract, countervail, neutralize, counterbalance]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "countervail":
   accommodate, adjust, amend, antagonize, atone for, balance,
   be antipathetic, be at cross-purposes, be inimical, be opposed to,
   beat against, cancel, clash, collide, compensate, conflict,
   conflict with, confute, contradict, contrapose, contrast with,
   contravene, controvert, coordinate, correct, counter, counteract,
   counterattack, counterbalance, countercheck, counterpoise,
   counterpose, counterweigh, counterwork, cross, equalize, equate,
   equiponderate, even, even up, fit, foil, frustrate, go against,
   go counter to, integrate, interfere with, join the opposition,
   juxtapose in opposition, level, lock horns, make up, meet head-on,
   militate against, not abide, offset, oppose, oppugn, outweigh,
   overcome, play at cross-purposes, poise, proportion, protest,
   rectify, redeem, resist, run against, run counter to, set off,
   square, square up, strike a balance, surmount, swim upstream,
   take issue with, thwart, traverse, vote against, withstand,
   work against