a reverse movement or tendency; an action in a reverse direction or manner.
movement in the direction of political conservatism or extreme rightism.
action in response to some influence, event, etc.: the nation's reaction to the president's speech.
Physiology. action in response to a stimulus, as of the system or of a nerve, muscle, etc.
the action caused by the resistance to another action.
a return to the opposite physical condition, as after shock, exhaustion, or chill.
Bacteriology, Immunology. the specific cellular response to foreign matter, as in testing for allergies.
Chemistry. the reciprocal action of chemical agents upon each other; chemical change.
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.
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.
Commerce. a decline in the market after an advance in prices.

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

reactional, adjective
reactionally, adverb
antireaction, adjective, noun
counterreaction, noun
nonreaction, noun
superreaction, noun Unabridged
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World English Dictionary
reaction (rɪˈækʃən)
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
 a.  Compare side effect any effect produced by the action of a drug, esp an adverse effect
 b.  any effect produced by a substance (allergen) to which a person is allergicthe simultaneous equal and opposite force that acts on a body whenever it exerts a force on another body
6.  chemical reaction short for nuclear reaction
7.  stock exchange a sharp fall in price interrupting a general rise
usage  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 10th Edition
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Word Origin & History

1611, from re- "again, anew" + action (q.v.). Modeled on Fr. réaction, older It. reattione, from M.L. reactionem (nom. reactio), from L.L. react-, pp. stem of reagere "react," from re- "back" + agere "to do, act" (see
act). 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. The verb react is attested from 1644.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

reaction re·ac·tion (rē-āk'shən)

  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
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American Heritage
Science Dictionary
reaction   (rē-āk'shən)  Pronunciation Key 
  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.


The American Heritage® Science Dictionary
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