Definition of: reaction

(rē·ak′shən) noun
1. Reverse or return action; tendency toward a former or reversed state of things; especially, a trend toward an earlier social, political, or economic policy or condition.
2. Physiol. Contrary action or reversed effects following a stimulus; a reflex action.
3. Psychol. The partial or total response made to any kind or degree of stimulation.
4. Physics In the second law of motion, the equal and opposite force exerted on an agent by the body acted upon.
5. Chem. The mutual action of substances subjected to chemical change, or some distinctive result of such action.
6. Biol. The effect upon any organism or any of its parts made by the introduction of any foreign substance for diagnostic or therapeutic purposes, or for testing, immunizing, etc.
—re·ac′tive adjective