re·ac·tion 
(rē-ăkshən)
(rē-ăkshən)n.
1.
a. A response to a stimulus.
b. The state resulting from such a response.
2. A reverse or opposing action.
3.
a. A tendency to revert to a former state.
b. Opposition to progress or liberalism; extreme conservatism.
4. Chemistry A change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.
5. Physics A nuclear reaction.
6. Physics An equal and opposite force exerted by a body against another body that is exerting a force on it.
7. The response of cells or tissues to an antigen, as in a test for immunization.