To counteract means to take action that reduces or stops the effect of something harmful or unwanted. When you counteract something, you're working against it to cancel it out or weaken its impact.

If you eat too much salty food and feel thirsty, drinking water helps counteract the salt's effects on your body. When engineers design a bridge, they must counteract the force of gravity and wind so the bridge doesn't collapse. A hockey goalie tries to counteract the other team's offense by blocking their shots.

The word suggests a purposeful response to a problem. You actively do something to oppose it rather than ignoring the problem or hoping it goes away. Medicine often works by counteracting harmful processes in the body. For example, an antibiotic counteracts a bacterial infection, and an antidote counteracts poison.

Teachers might counteract student boredom by making lessons more interactive and engaging. A chess player might counteract their opponent's aggressive strategy by building strong defenses. The key idea is that when you counteract something, you're taking specific action designed to reduce or eliminate the unwanted effect.