counteract in WordNet English dictionary

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