A response to a stimulus. The state resulting from such a response. A reverse or opposing action. A tendency to revert to a former state. Opposition to progress or liberalism; extreme conservatism. Chemistry A change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances. Physics A nuclear reaction. Physics An equal and opposite force exerted by a body against a force acting upon it. The response of cells or tissues to an antigen, as in a test for immunization.