reaction

American  

[ree-ak-shuhn] / 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. Also called chemical reaction.  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.

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reaction British  

/ 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

6. short for chemical reaction nuclear reaction

7. stock exchange a sharp fall in price interrupting a general rise

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reaction Scientific  

/ rē-ăk′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.

4. See more at Newton's laws of motion

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

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

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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

Other Word Forms

• antireaction
  adjective
• counterreaction
  noun
• nonreaction
  noun
• reactional
  adjective
• reactionally
  adverb
• superreaction
  noun

Etymology

Origin of reaction

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

Explanation

A reaction is an action taken in response to something. If you're telling your parents that you want to move out, you'll see by their reaction that they're sad about it. A reaction is often a physical in nature. A chemical reaction describes the way a chemical behaves when combined with another substance. The way your body responds to a medication or external influence is a physical reaction. Think of the way cold air causes you to get goose bumps on your arms. It can also describe something more emotional, for instance a person’s reaction of laughter when told a funny story.

Example Sentences

“Basically, I’m wondering if the reaction is over, or, or if upon stopping the cooling mechanism, if it would re-initiate itself.”

From Los Angeles Times • May 23, 2026

The strike has drawn a strong reaction from top Russian officials, with President Vladimir Putin ordering the army to prepare a response.

From Barron's • May 23, 2026

“The universal reaction was shock,” said Lesa Lessard Pearson, chair of the Select Board in Belchertown, a town of 15,000 in western Massachusetts.

From The Wall Street Journal • May 23, 2026

“The reaction is releasing heat. That’s going to initiate more reaction to happen, so it might even cascade.”

From Los Angeles Times • May 23, 2026

It all sounds easy enough, but in practice, starting that chain reaction is an extremely tricky process.

From "Meltdown" by Deirdre Langeland

Related Words

• answer
• attitude
• backlash
• feedback
• feeling
• opinion
• reception
• reply
• return