- an action performed or a feeling experienced in response to a situation or event: Carrie’s immediate reaction was one of relief.
- (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 chemical process in which two or more substances act mutually on each other and are changed into different substances, or one substance changes into two or more other substances.
- Physics an analogous transformation of atomic nuclei or other particles.
- a mode of thinking or behaving that is deliberately different from previous modes of thought and behavior: the work of these painters was a reaction against fauvism.
- opposition to political or social progress or reform: the institution is under threat from the forces of reaction.
- Physics repulsion or resistance exerted in opposition to the impact or pressure of another body; a force equal and opposite to the force giving rise to it.