1 : to act or behave in response to something Firefighters reacted quickly to the situation. Hint: In this sense, react is usually used with against. Workers reacted against unfair treatment. 3 : to go through a chemical reaction Acids react with bases to form salt.

1. to act in response to an agent, influence, stimulus, etc.: to react to a medicine; reacted to the noise by jumping. 2. to act reciprocally upon each other, as two things. 3. to act in a reverse direction or manner, esp. so as to return to a prior condition. 4. to act in opposition, as against some force.