Responding to something that happens, usually quickly and without a lot of planning.
From the Latin 're-' (back, again) combined with 'agere' (to do or act). The prefix 're-' suggests a backward or responsive action—you're acting in response to something else. This scientific term entered common speech in the 17th century.
In chemistry, a reaction is when substances interact and transform into something new—but we've extended this metaphor to emotions too, which is why we say we 'react' to surprising news, even though it's really just our brain responding!
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