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