1.a response that reveals a person's feelings or attitude;
he was pleased by the audience's reaction to his performance
2.a bodily process occurring due to the effect of some foregoing stimulus or agent;
a bad reaction to the medicine
3.(chemistry) a process in which one or more substances are changed into others;
there was a chemical reaction of the lime with the ground water
4.an idea evoked by some experience;
his reaction to the news was to start planning what to do
5.doing something in opposition to another way of doing it that you don't like;
his style of painting was a reaction against cubism
6.extreme conservatism in political or social matters;
the forces of reaction carried the election
7.(mechanics) the equal and opposite force that is produced when any force is applied to a body;
every action has an equal and opposite reaction