'Counteract' definitions:

Definition of 'counteract'

(from WordNet)
verb
Act in opposition to [syn: antagonize, antagonise, counteract]
verb
Oppose or check by a counteraction [syn: countercheck, counteract]
verb
Oppose and mitigate the effects of by contrary actions; "This will counteract the foolish actions of my colleagues" [syn: counteract, countervail, neutralize, counterbalance]
verb
Destroy property or hinder normal operations; "The Resistance sabotaged railroad operations during the war" [syn: sabotage, undermine, countermine, counteract, subvert, weaken]

Definition of 'Counteract'

From: GCIDE
  • 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]

Synonyms of 'counteract'

From: Moby Thesaurus