A reaction is what happens in response to something else, such as how a person feels or behaves after an event, or how chemicals change when they come into contact. It can also mean resistance to change in politics or society.
It comes from Latin “reactio,” from “re-” (back) and “actio” (action), meaning “a driving back” or “opposition.” The emotional and chemical senses both developed from this basic idea of something responding to a push.
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