[ree-ak-shuh n]


Origin of reaction

1635–45; re- + action, modeled on react
Related formsre·ac·tion·al, adjectivere·ac·tion·al·ly, adverban·ti·re·ac·tion, adjective, nouncoun·ter·re·ac·tion, nounnon·re·ac·tion, nounsu·per·re·ac·tion, noun Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for reaction

Contemporary Examples of reaction

Historical Examples of reaction

British Dictionary definitions for reaction



a response to some foregoing action or stimulus
the reciprocal action of two things acting together
opposition to change, esp political change, or a desire to return to a former condition or system
a response indicating a person's feelings or emotional attitude
  1. any effect produced by the action of a drug, esp an adverse effectCompare side effect
  2. any effect produced by a substance (allergen) to which a person is allergic, the simultaneous equal and opposite force that acts on a body whenever it exerts a force on another body
stock exchange a sharp fall in price interrupting a general rise
Derived Formsreactional, adjective


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

Word Origin and History for reaction

"action in resistance or response to another action or power," 1610s, from re- "again, anew" + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), noun of action formed in Late Latin from past participle stem of Latin reagere "react," from re- "back" + agere "to do, act" (see act (v.)).

Originally scientific; physiological sense is attested from 1805; psychological sense first recorded 1887; general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.

Online Etymology Dictionary, © 2010 Douglas Harper

reaction in Medicine




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.

reaction in Science



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.