Definitions of reaction:

• noun:   doing something in opposition to another way of doing it that you don't like

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

• noun:   a bodily process occurring due to the effect of some foregoing stimulus or agent

  Example: "A bad reaction to the medicine"

• noun:   an idea evoked by some experience

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

• noun:   extreme conservatism in political or social matters

  Example: "The forces of reaction carried the election"

• noun:   a response that reveals a person's feelings or attitude

  Example: "He was pleased by the audience's reaction to his performance"

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

  Example: "Every action has an equal and opposite reaction"

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

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