1 : the way someone acts or feels in response to something that happens, is said, etc. [count] ▪ 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. [noncount] ▪ There hasn't yet been much reaction to the announcement. ▪ The newspaper received many angry letters in reaction to [=in response to] the editorial. 2 : an action or attitude that shows disagreement with or disapproval of someone or something [count] — usually singular — often + against ▪ The modernist movement in literature was a reaction against traditional literary conventions. [noncount] ▪ There has been strong reaction against the government's policies. 3 reactions [plural] : the ability to act and move quickly in order to avoid sudden danger
▪ A good driver has quick reactions. ▪ The speed of his reactions saved his life. 4 medical : an occurrence in which your body is affected by a drug, food, etc., in usually a bad way [count] ▪ I had/suffered a (bad) reaction to the medicine. ▪ Something she ate caused/triggered an allergic reaction. [noncount] ▪ There is a risk of adverse reaction to the medicine. 5 a chemistry : a chemical change that occurs when two or more substances combine to form a new substance [count] ▪ The reaction of hydrogen with oxygen makes water. ▪ chemical reactions [noncount] ▪ a substance produced by chemical reaction
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; — see also chain reaction 6 [count, noncount] physics : a physical force that opposes the action of an equal and opposite force
▪ For every action, there is an equal and opposite reaction. 7 [noncount] formal : a strong tendency or desire to oppose new political or social ideas
▪ They have had to fight against the forces of reaction.