Meaning of “reaction” - Learner’s Dictionary

reaction

noun uk us /riˈækʃən/
CAUSED BY SOMETHING [ C, U ]

B2 something you say, feel, or do because of something that has happened:

What was his reaction to the news?
reactions mainly UK

the ability to move quickly when something suddenly happens:

Drivers need to have quick reactions.
CHANGE [ no plural ]

a change in the way people behave or think because they do not agree with the way people behaved or thought in the past:

In art, there was a reaction against Realism.
BAD EFFECT [ C ]

B2 an unpleasant feeling or illness caused by something you have eaten or used on your body:

A number of people have had a bad reaction to this drug.
SUBSTANCES [ C ]

a change that happens when two substances are put together:

a chemical reaction
→ See also chain reaction

(Definition of “reaction” from the Cambridge Learner’s Dictionary © Cambridge University Press)