(1)   Doing something in opposition to another way of doing it that you don't like
"His style of painting was a reaction against cubism"
(2)   A bodily process occurring due to the effect of some foregoing stimulus or agent
"A bad reaction to the medicine"
"His responses have slowed with age"
(3)   An idea evoked by some experience
"His reaction to the news was to start planning what to do"
(4)   Extreme conservatism in political or social matters
"The forces of reaction carried the election"
(5)   A response that reveals a person's feelings or attitude
"He was pleased by the audience's reaction to his performance"
"John feared his mother's reaction when she saw the broken lamp"
(6)   (mechanics) the equal and opposite force that is produced when any force is applied to a body
"Every action has an equal and opposite reaction"
(7)   (chemistry) a process in which one or more substances are changed into others
"There was a chemical reaction of the lime with the ground water"


From re-, again, + action, from Latin actio, noun of action form from perfect passive participle actus, done, from verb agere, do.


  1. An action or statement in response to a stimulus or other event
    The announcement of the verdict brought a violent reaction.
    You were in the coutroom. What is your reaction?
    When I last tried to eat strawberries I had a terrible allergic reaction.
  2. A transformation in which one or more substances is converted into another by combination or decomposition
    In this reaction, the acid and base will neutralize each other, producing a salt.

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