v. re·act·ed, re·act·ing, re·acts
1. To act in response to or under the influence of a stimulus or prompting: reacted strongly to the sarcastic tone of the memorandum.
2. To act in opposition to a former condition or act: composers who reacted against romanticism.
3. To act reciprocally or in return.
4. Chemistry To undergo a reaction: Methane reacts with hydroxyl to produce formaldehyde.
To cause (a substance or substances) to undergo a reaction.
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.