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