verb

• (intransitive) To act in response.
  "How did she react to the news?"
• (transitive, now rare) To act or perform a second time; to do over again; to reenact.
  "It is somewhat extraordinary, that the offence for which James II, was expelled, that of setting up power by assumption, should be re-acted, under another shape and form, by the parliament that expelled him."
• (physics, intransitive) To return an impulse or impression; to resist the action of another body by an opposite force
  "Every body reacts on the body that impels it from its natural state."
• (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.

noun

• (Internet) An emoji used to express a reaction to a post on social media.
  "Sad reacts only"