View synonyms for reaction


[ ree-ak-shuhn ]


  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.


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


/ rē-ăkshən /

  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. 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.
  3. A response to a stimulus, such as a reflex.
  4. The response of cells or tissues to an antigen, as in a test for immunization.

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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
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Derived Forms

  • reˈactional, adjective
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Other Words From

  • re·action·al adjective
  • re·action·al·ly adverb
  • anti·re·action adjective noun
  • counter·re·action noun
  • nonre·action noun
  • super·re·action noun
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Word History and Origins

Origin of reaction1

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

Despite some evidence that excipients are responsible for drug reactions, the amount of each excipient added to each drug is not reported for nearly half of biological medicines.

Using water from different sources won’t change the reaction.

We decided to test the main hypothesis, which assumes most consumers will have a negative reaction.

Huang pointed to the government’s strict media censorship, which could block reports about side effects or bad reactions to the vaccine.

From Vox

In the meantime, the company will review all safety data collected so far, and researchers running the nine other phase 3 coronavirus vaccine trials will comb through their data to look for similar cases of adverse reactions.

We haven't had any real fan reaction yet, but our collective fingers are crossed.

Or how kicking a reaction-less eunuch in the crotch over and over again is comedy.

Your general reaction runs along the lines of: “When will these geezers give it up and go for a mall walk or something?”

Jundullah and Jaish ul Adl sprang up “in reaction to that kind of oppression,” he said.

No one likes it when their sandcastle is knocked over, but his reaction is a bit, err, extreme.

While this reaction lasted he laughed away the evidence, and honestly believed he was exaggerating trifles.

Ordinarily the diazo appears a little earlier than the Widal reaction—about the fourth or fifth day—but it may be delayed.

For Lettice—the tender woman of his first acquaintance—had obviously experienced a moment of reaction.

When applied to the diagnosis of typhoid fever, the phenomenon is known as the Widal reaction.

As yet, it is the only agglutination reaction which has any practical value for the practitioner.