- To counteract is to act in opposition to or against some force in order to neutralize the force.
When you try to grab a rope and your friend pulls it away from you in the other direction, this is an example of a time when your friend counteracts your efforts.
transitive verbcoun·ter·act·ed, coun·ter·act·ing, coun·ter·acts
(third-person singular simple present counteracts, present participle counteracting, simple past and past participle counteracted)
From counter- + act.