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 a force acting upon it.
7. The response of cells or tissues to an antigen, as in a test for immunization.
8. Psychology. A pattern of behavior constituting a mental disorder or personality type.