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reaction

[ree-ak-shuh n] /riˈæk ʃən/
noun
1.
a reverse movement or tendency; an action in a reverse direction or manner.
2.
movement in the direction of political conservatism or extreme rightism.
3.
action in response to some influence, event, etc.:
the nation's reaction to the president's speech.
4.
Physiology. action in response to a stimulus, as of the system or of a nerve, muscle, etc.
5.
Medicine/Medical.
  1. the action caused by the resistance to another action.
  2. a return to the opposite physical condition, as after shock, exhaustion, or chill.
6.
Bacteriology, Immunology. the specific cellular response to foreign matter, as in testing for allergies.
7.
Chemistry. the reciprocal action of chemical agents upon each other; chemical change.
8.
Also called nuclear reaction. Physics. a process in which a nucleus that is bombarded by a photon, particle, or other nucleus, emits a nucleon, alpha particle, or the like, without a significant change in its atomic weight.
9.
Mechanics. the instantaneous response of a system to an applied force, manifested as the exertion of a force equal in magnitude but opposite in direction to the applied force.
10.
Commerce. a decline in the market after an advance in prices.
Origin
1635-1645
1635-45; re- + action, modeled on react
Related forms
reactional, adjective
reactionally, adverb
antireaction, adjective, noun
counterreaction, noun
nonreaction, noun
superreaction, noun
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Examples from the web for reaction
  • One obvious risk is of a political reaction against the philanthropic rich.
  • Even a small political shock could push over a domino causing a chain reaction that leads to one outcome or the other.
  • Anaphylaxis is a severe, whole-body allergic reaction to a chemical that has become an allergen.
  • reaction to the long-awaited budget details was swift.
  • The reaction from our chickens was nothing short of amazing.
  • At high enough densities, this chain reaction of neutron-induced fission can produce a nuclear explosion.
  • But his third reaction was more positive, or desperate.
  • Often the allergen and the immune system become increasingly antagonistic, and the reaction worsens.
  • It so happens that my personal reaction was in line with what the designer intended to convey.
  • The control rods absorb neutrons to prevent the reaction with uranium that causes fission.
British Dictionary definitions for reaction

reaction

/rɪˈækʃən/
noun
1.
a response to some foregoing action or stimulus
2.
the reciprocal action of two things acting together
3.
opposition to change, esp political change, or a desire to return to a former condition or system
4.
a response indicating a person's feelings or emotional attitude
5.
(med)
  1. any effect produced by the action of a drug, esp an adverse effect Compare 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
7.
(stock exchange) a sharp fall in price interrupting a general rise
Derived Forms
reactional, adjective
Usage note
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
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Word Origin and History for reaction
n.

"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
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reaction in Medicine

reaction re·ac·tion (rē-āk'shən)
n.

  1. A response of an organism or living tissue to a stimulus.

  2. The state resulting from such a response.

  3. A chemical change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.

  4. The response of cells or tissues to an antigen, as in a test for immunization.

  5. 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.
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reaction in Science
reaction
  (rē-āk'shən)   
  1. 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.

  2. See nuclear reaction.

  3. 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.

  4. A response to a stimulus, such as a reflex.

  5. The response of cells or tissues to an antigen, as in a test for immunization.


The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Slang definitions & phrases for reaction

reaction

Related Terms

gut reaction


The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D.
Copyright (C) 2007 by HarperCollins Publishers.
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Nearby words for reaction