noun (n)

• (chemistry) a process in which one or more substances are changed into others(noun.process)

  Example:
  There was a chemical reaction of the lime with the ground water.
  

  Synonym:
  chemical reaction
  

  source: wordnet30
• an idea evoked by some experience(noun.cognition)

  Example:
  His reaction to the news was to start planning what to do.
  

  source: wordnet30
• a bodily process occurring due to the effect of some antecedent stimulus or agent(noun.act)

  Example:
  A bad reaction to the medicine.
  

  Synonym:
  response
  

  source: wordnet30
• (mechanics) the equal and opposite force that is produced when any force is applied to a body(noun.phenomenon)

  Example:
  Every action has an equal and opposite reaction.
  

  source: wordnet30
• a response that reveals a person's feelings or attitude(noun.phenomenon)

  Example:
  He was pleased by the audience's reaction to his performance.
  John feared his mother's reaction when she saw the broken lamp.
  

  source: wordnet30
• extreme conservatism in political or social matters(noun.cognition)

  Example:
  The forces of reaction carried the election.
  

  source: wordnet30
• doing something in opposition to another way of doing it that you don't like(noun.act)

  Example:
  His style of painting was a reaction against cubism.
  

  source: wordnet30
• Any action in resisting other action or force; counter tendency; movement in a contrary direction; reverse action.(noun)

  source: webster1913
• A regular or characteristic response to a stimulation of the nerves.(noun)

  source: webster1913