verb (v)

• act in opposition to(verb.social)

  Synonym:
  antagonise, antagonize
  

  source: wordnet30
• oppose or check by a counteraction(verb.social)

  Synonym:
  countercheck
  

  source: wordnet30
• oppose and mitigate the effects of by contrary actions(verb.social)

  Example:
  This will counteract the foolish actions of my colleagues.
  

  Synonym:
  counterbalance, countervail, neutralize
  

  source: wordnet30
• destroy property or hinder normal operations(verb.social)

  Synonym:
  countermine, sabotage, subvert, undermine, weaken
  

  source: wordnet30
• 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.(verb)

  source: webster1913