Definitions from Wiktionary ( )

▸ verb:  (intransitive) To act in response.
▸ verb:  (transitive, now rare) To act or perform a second time; to do over again; to reenact.
▸ verb:  (physics, intransitive) To return an impulse or impression; to resist the action of another body by an opposite force
▸ verb:  (chemistry, intransitive) To act upon each other; to exercise a reciprocal or a reverse effect, as two or more chemical agents; to act in opposition.
▸ verb:  (chemistry, transitive) To cause chemical agents to react; to cause one chemical agent to react with another.
▸ verb:  (Internet, intransitive) To post a reaction (icon or emoji indicating how one feels about a posted message).
▸ noun:  (Internet) An emoji used to express a reaction to a post on social media.

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