To counteract is to act in opposition to or against some force in order to neutralize the force.(verb)
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.
See counteract in Webster's New World College Dictionary
See counteract in American Heritage Dictionary 4
transitive verb coun·ter·act·ed, coun·ter·act·ing, coun·ter·acts
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