1something done, felt, or thought in response to a situation or event:my immediate reaction was one of relief [mass noun]:prices fell in reaction to intense competition
(reactions) a person’s ability to respond physically and mentally to external stimuli:a skilled driver with quick reactions
an adverse physiological response to a substance that has been breathed in, ingested, or touched:such allergic reactions as hay fever and asthma
a mode of thinking or behaving that is deliberately different from previous modes of thought and behaviour:the work of these painters was a reaction against Fauvism
[mass noun] opposition to political or social progress or reform:the institution is under threat from the forces of reaction
2a chemical process in which substances act mutually on each other and are changed into different substances, or one substance changes into other substances:a chemical reaction caused by a build-up of particular sodium salts
Physics an analogous transformation of atomic nuclei or other particles:reactions between photons and electrically charged atomic particles
3 [mass noun] Physics a force exerted in opposition to an applied force:the law of action and reaction
