(n)chemical reaction, reaction(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”
(n)reaction, responsea bodily process occurring due to the effect of some antecedent stimulus or agent“a bad reaction to the medicine”, “his responses have slowed with age”
(n)reaction(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”
(n)reactiona 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”