Learner's definition of REACTION
1 : the way someone acts or feels in response to something that happens, is said, etc.
Their reaction to the news was positive.
When I told him what happened, his immediate reaction was one of shock.
The announcement produced an angry/stunned reaction from most of the people at the meeting.
My first reaction when I met him was not to trust him.
There has been a mixed reaction [=people have reacted in different ways] to the new policy.
2 : an action or attitude that shows disagreement with or disapproval of someone or something
[count]— usually singular— often + against
3 reactions [plural] : the ability to act and move quickly in order to avoid sudden danger 4 medical : an occurrence in which your body is affected by a drug, food, etc., in usually a bad way
5 a chemistry : a chemical change that occurs when two or more substances combine to form a new substance
[noncount] b [count, noncount], physics : a process in which the nucleus of an atom is changed by being split apart or joined with the nucleus of another atom
— called also nuclear reaction
6 [count, noncount], physics : a physical force that opposes the action of an equal and opposite force 7 [noncount], formal : a strong tendency or desire to oppose new political or social ideas