a. A response to a stimulus.
b. The state resulting from such a response.
2. A reverse or opposing action.
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.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.