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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.
Medicine/Medical.

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.
Contemporary Examples

My first reaction, and my second, third and fourth, was the howl of pain.
William F. Buckley’s Flip-Flop Richard Brookhiser July 8, 2009

Bundy offered his own view: “My five-second reaction is to have the small war but not today.”
The Commander-in-Chief Test: What the Cuban Missile Crisis Tells Us About JFK David G. Coleman October 15, 2012

The reaction of Republican Senator Mark Kirk of Illinois was typical.
Iran’s Supreme Leader Ali Khamenei Eyes U.S. Shutdown, Expresses Doubt Over Nuclear Negotiations Christopher Dickey October 5, 2013

That felt like cheating, I imagined my own reaction to reading a book and then finding such an addendum at the end.
Speak, Faulty Memory: Why Memoir Writing Is Harder Than You Think Dave Bry April 2, 2013

Which is one reason why the reaction to this study from Obamacare’s supporters has frankly been a bit disappointing.
Study: Giving People Government Health Insurance May Not Make them Any Healthier Megan McArdle April 30, 2013

Historical Examples

It must be remembered that following the war there was a reaction against Seward.
Lincoln Nathaniel Wright Stephenson

The business world reflects the disturbance of war’s reaction.
United States Presidents’ Inaugural Speeches Various

“You can bet their first reaction would be to dismantle the ship at sight,” Garth informed him.
Deepfreeze Robert Donald Locke

Her eyes were blazing with an anger the more fierce in that some of it was reaction.
The Leopard Woman Stewart Edward White

Not that the reaction is so definitely formulated in the moment of experience; but this is something of what is felt.
The Gate of Appreciation Carleton Noyes

noun
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
(med)

any effect produced by the action of a drug, esp an adverse effect Compare side effect
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

short for chemical reaction, nuclear reaction
(stock exchange) a sharp fall in price interrupting a general rise
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.

reaction re·ac·tion (rē-āk’shən)
n.

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.

reaction
(rē-āk’shən)

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.

Related Terms

gut reaction