Learner's definition of REACT
to behave or change in a particular way when something happens, is said, etc.
When I told her what happened, she reacted with anger.
I didn't expect him to react that way.
The firefighters reacted quickly when they heard the alarm.
— often + to
see also overreact
of a chemical substance
to change after coming into contact with another substance
— often + with
— often + together
to be affected by a drug, food, etc., in usually a bad way
react against (someone or something)
to do things that are opposed to (something or someone that you disagree with)