Definition of "reaction" []

• A response to a stimulus. (noun)
• The state resulting from such a response. (noun)
• A reverse or opposing action. (noun)
• A tendency to revert to a former state. (noun)
• Opposition to progress or liberalism; extreme conservatism. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Such a reaction is called a _reversible reaction_."
• "So the first answer to your question is that there is no chain reaction in Earth’s core because there probably isn’t enough uranium there to initiate a chain reaction¢’‚¬?"
• "Of particular note was the elimination of the word reaction from the diagnostic labels."