Definition of "react"

(intransitive) To act in response.  examples

(transitive, now rare) To act or perform a second time; to do over again; to reenact.  quotations

(physics, intransitive) To return an impulse or impression; to resist the action of another body by an opposite force  examples

(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.  examples

(chemistry, transitive) To cause chemical agents to react; to cause one chemical agent to react with another.  examples

(Internet, intransitive) To post a reaction (icon or emoji indicating how one feels about a posted message).  examples

(Internet) An emoji used to express a reaction to a post on social media.  examples