[ ree-ak-shuhn ]
/ riˈæk ʃən /
See synonyms for: reaction / reactions on




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Origin of reaction

First recorded in 1635–45; re- + action, modeled on react

OTHER WORDS FROM reaction Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

Example sentences from the Web for reaction

British Dictionary definitions for reaction

/ (rɪˈækʃən) /


Derived forms of reaction

reactional, adjective

usage for reaction

Reaction is used to refer both to an instant response (her reaction was one of amazement) and to a considered response in the form of a statement (the Minister gave his reaction to the court's decision). Some people think this second use is incorrect
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for reaction

[ rē-ăkshən ]


A response of an organism or living tissue to a stimulus.
The state resulting from such a response.
A chemical change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.
The response of cells or tissues to an antigen, as in a test for immunization.
A pattern of behavior constituting a mental disorder or personality type.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for reaction

[ rē-ăkshən ]

A rearrangement of the atoms or molecules of two or more substances that come into contact with each other, resulting in the formation of one or more new substances. Chemical reactions are caused by electrons of one substance interacting with those of another. The reaction of an acid with a base, for example, results in the creation of a salt and water. Some, but not all, reactions can be reversed.
See nuclear reaction.
An action that results directly from or counteracts another action, especially the change in a body's motion as a result of a force applied to it. Some reactions counteract forces and are not readily apparent. When an object rests on a surface, such as a table, for example, the downward force it applies to the surface is counteracted by an equal but upwards force, or reaction, applied by the surface. See more at Newton's laws of motion.
A response to a stimulus, such as a reflex.
The response of cells or tissues to an antigen, as in a test for immunization.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.