Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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