Definition of reaction

• a response that reveals a persons feelings or attitude; "he was pleased by the audiences reaction to his performance"; "John feared his mothers reaction when she saw the broken lamp"
• 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"
• (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"