verb (v)

• show a response or a reaction to something(verb.cognition)

  Synonym:
  respond
  

  source: wordnet30
• act against or in opposition to(verb.social)

  Example:
  She reacts negatively to everything I say.
  

  Synonym:
  oppose
  

  source: wordnet30
• undergo a chemical reaction; react with another substance under certain conditions(verb.change)

  Example:
  The hydrogen and the oxygen react.
  

  source: wordnet30
• To act or perform a second time; to do over again; as, to react a play; the same scenes were reacted at Rome.(verb)

  source: webster1913
• To return an impulse or impression; to resist the action of another body by an opposite force; as, every body reacts on the body that impels it from its natural state.(verb)

  source: webster1913