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reaction (n.)

"action in resistance or response to another action or power," 1640s, from re- "back, again, anew" + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), a noun of action formed in Late Latin from the past-participle stem of Latin reagere "react," from re- "back" + agere "to do, perform."

Originally a word in physics and dynamics. In chemistry, "mutual or reciprocal action of chemical agents upon each other," by 1836. The general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.

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Definitions of reaction from WordNet

reaction (n.)
(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
Synonyms: chemical reaction
reaction (n.)
an idea evoked by some experience;
his reaction to the news was to start planning what to do
reaction (n.)
a bodily process occurring due to the effect of some antecedent stimulus or agent;
a bad reaction to the medicine
Synonyms: response
reaction (n.)
(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
reaction (n.)
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
reaction (n.)
extreme conservatism in political or social matters;
the forces of reaction carried the election
reaction (n.)
doing something in opposition to another way of doing it that you don't like;
his style of painting was a reaction against cubism
From wordnet.princeton.edu