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re·ac·tion

(rē-ăkshən)

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.

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:

Indo-European Roots

Semitic Roots

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.

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Indo-European & Semitic Roots Appendices

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