The Collaborative International Dictionary of English v.0.48:

Counteract \Coun`ter*act"\ (koun`t?r-?kt"), v. t. [imp. & p. p.
   Counteracted; p. pr. & vb. n. Counteracting.]
   To act in opposition to; to hinder, defeat, or frustrate, by
   contrary agency or influence; as, to counteract the effect of
   medicines; to counteract good advice.
   [1913 Webster]

WordNet (r) 3.0 (2006):

counteract
    v 1: act in opposition to [syn: antagonize, antagonise,
         counteract]
    2: oppose or check by a counteraction [syn: countercheck,
       counteract]
    3: oppose and mitigate the effects of by contrary actions; "This
       will counteract the foolish actions of my colleagues" [syn:
       counteract, countervail, neutralize, counterbalance]
    4: destroy property or hinder normal operations; "The Resistance
       sabotaged railroad operations during the war" [syn:
       sabotage, undermine, countermine, counteract,
       subvert, weaken]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "counteract":
   annul, antagonize, baffle, balance, balk, be antipathetic,
   be at cross-purposes, be inimical, be opposed to, beat against,
   blast, brave, bump heads, cancel, cancel out, challenge, checkmate,
   circumvent, clash, collide, conflict, conflict with, confound,
   confront, confute, contend, contradict, contrapose, contraposit,
   contrast, contrast with, contravene, controvert, correct, counter,
   counterattack, counterbalance, countercheck, countermand,
   counterpoise, counterpose, countervail, counterweigh, counterwork,
   cross, dash, defeat, defy, destroy, discomfit, disconcert,
   discountenance, dish, disrupt, elude, equiponderate, face, fix,
   flummox, foil, front, frustrate, go against, go counter to,
   interfere with, join the opposition, juxtapose in opposition,
   knock the chocks, lie opposite, lock horns, match, meet head-on,
   militate against, mitigate, negate, negative, neutralize, nonplus,
   not abide, nullify, offset, oppose, oppugn, perplex,
   play at cross-purposes, polarize, pose against, protest,
   put in opposition, rectify, redress, resist, right, ruin,
   run against, run counter to, sabotage, scotch, set off,
   set over against, spike, spoil, square, square up, stand opposed,
   stand opposite, stonewall, stump, subtend, swim upstream,
   take issue with, thwart, traverse, upset, vote against, withstand,
   work against