reaction

reaction — noun

1. your way of behaving, feeling, or acting that follows directly from an event or

1.nounB1
Definition

your way of behaving, feeling, or acting that follows directly from an event or situation — for example, feeling shock after hearing bad news, or clapping when a performance finishes.

Example

Tamar's immediate reaction to the job offer was one of surprise and delight.

immediate reaction + to + noun phrase

The city council's reaction to the housing shortage was to approve new building permits.

reaction + to + noun + verb pattern

Common collocations
  • immediate reaction
  • strong reaction
  • mixed reaction
  • first reaction
  • gut reaction
  • emotional reaction
  • natural reaction
  • common reaction
Synonyms
  • response

    more deliberate and often implies thought; 'reaction' can be more automatic

  • reply

    limited to words or written answers, not feelings or actions

  • feedback

    evaluative reaction given to help someone improve

Antonyms

Grammar Patterns

reaction + to + noun/pronoun/gerund

in reaction + to + noun

as a reaction + to + noun

there + be + a reaction + to + noun

Usage Note

Often followed by 'to' to indicate the cause. The verb pattern varies: a reaction can be a feeling, a word, or an action.

Common Mistakes

What was your reaction about the movie?
What was your reaction to the movie?
use 'to', not 'about', after 'reaction'.
I had a reaction of anger
My reaction was one of anger' or 'I reacted with anger
'reaction of' is rare and unnatural in most contexts.

2. a physical change in the body that shows you have a negative sensitivity to a su

2.nounB2
Definition

a physical change in the body that shows you have a negative sensitivity to a substance such as food, medicine, or pollen — examples include rashes, swelling, breathing difficulty, or an itchy feeling after eating something you are allergic to.

Example

Marta had a severe allergic reaction to the shellfish and needed urgent medical attention.

allergic reaction + to + substance — medical collocation

Ravindra developed a red, itchy skin reaction three days after starting the penicillin treatment.

Common collocations
  • allergic reaction
  • adverse reaction
  • severe reaction
  • mild reaction
  • skin reaction
  • have a reaction to
  • experience a reaction
Synonyms
  • allergy

    the long-term condition, not the individual event; 'reaction' is a single episode

  • response

    neutral; 'reaction' in this sense implies negative sensitivity

  • side effect

    broader — includes any unintended effect, not just allergic ones

Grammar Patterns

reaction + to + noun (substance/formula)

have + a + reaction + to + noun

experience + a + reaction

Usage Note

Medical context. 'Reaction' can be mild (slight rash) or severe (anaphylaxis). 'Adverse reaction' is a formal medical term for any unwanted drug effect, not just allergy.

Common Mistakes

I had an allergy reaction to peanuts
I had an allergic reaction to peanuts
the adjective is 'allergic', not 'allergy'.
He reacted to the medicine' (when a bodily reaction is meant)
He had a reaction to the medicine
'have a reaction' is the natural noun form.

3. the speed with which someone can act or respond when something unexpected happen

3.nounB2
Definition

the speed with which someone can act or respond when something unexpected happens — used especially about physical reflexes, such as a driver braking to avoid a crash or a goalkeeper saving a shot.

Example

The driver's quick reaction prevented a serious accident on the icy highway.

quick reaction — adjective + reaction describing speed

A good goalkeeper needs excellent reactions to stop a fast-moving penalty kick.

Common collocations
  • quick reactions
  • fast reactions
  • excellent reactions
  • reaction time
  • reaction speed
  • good reactions
Synonyms
  • reflexes

    more specific to automatic physical responses; 'reactions' can include conscious decisions

  • quickness

    general speed of response, less specific than 'reactions'

Grammar Patterns

reaction + time(s)

have + adjective + reactions

quick/fast/good reactions

Usage Note

In this sense 'reactions' (plural) is common in British English ('She has good reactions'). 'Reaction time' (singular) is the standard term in sports science and psychology.

Common Mistakes

He has a very fast reaction' (meaning physical reflexes)
He has very fast reactions
use the plural when talking about physical response ability.

4. [chemistry] a change where substances interact and turn into new materials — for

4.nounB2
Definition

[chemistry] a change where substances interact and turn into new materials — for example, iron rusting when it meets oxygen and water, or an acid and a base combining to create salt and water.

Example

The students observed a chemical reaction when they mixed vinegar with baking soda.

chemical reaction — scientific collocation

Iron powder meeting oxygen in a hand warmer pack triggers a reaction that releases enough heat to warm your palms.

released during + reaction — prepositional common pattern

Common collocations
  • chemical reaction
  • chain reaction
  • nuclear reaction
  • trigger a reaction
  • speed up a reaction
  • reaction between
  • reversible reaction
  • exothermic reaction
Synonyms

Grammar Patterns

chemical + reaction

set off / trigger + a + reaction

reaction + between + noun + and + noun

Usage Note

General term for all chemical transformations. 'Chain reaction' describes a sequence where each step triggers the next, common in nuclear physics. 'Nuclear reaction' is a specific type involving atomic nuclei.

Common Mistakes

The chemicals reacted each other
The chemicals reacted with each other
'react' requires 'with'.

5. a way of behaving, thinking, or creating that is deliberately the opposite of a

5.nounC1
Definition

a way of behaving, thinking, or creating that is deliberately the opposite of a previous style, idea, or norm — for example, young people adopting a fashion style that rebels against older generations' tastes.

Example

The teenager's taste in music was a reaction against his parents' classical preferences.

reaction against — followed by 'against'

Filmmaker Chidi adopted a raw, hand-held camera style as a reaction to glossy Hollywood blockbusters.

as a reaction to — prepositional phrase

Common collocations
  • reaction against
  • as a reaction to
  • deliberate reaction
  • cultural reaction
  • political reaction
Synonyms
  • backlash

    stronger and more negative; implies anger or resentment from a group

  • rebellion

    more active and confrontational; 'reaction' can be quieter

Antonyms
  • acceptance

    embracing rather than opposing the existing norm

Grammar Patterns

reaction + against + noun/gerund

as a reaction + to + noun

Usage Note

Frequently followed by 'against' (direct opposition) or 'to' (general response context). This sense carries a deliberate, sometimes ideological tone of rejection.

Common Mistakes

His choice was a reaction of the current trend
His choice was a reaction against the current trend
use 'reaction against', not 'reaction of'.

6. the position that opposes reform and tries to keep society and politics from cha

6.nounC1
Definition

the position that opposes reform and tries to keep society and politics from changing — for example, organisations that fight against new laws about equal rights or environmental standards.

Example

The election result was seen as a reaction against the government's education reforms.

reaction against — followed by 'against' in political context

The governor's new tax on farmers triggered a reaction from growers who blocked the highway with their tractors.

triggered a reaction from + [group] who [concrete action]

Common collocations
  • reaction against
  • forces of reaction
  • political reaction
  • social reaction
Synonyms
  • conservatism

    broader; includes moderate resistance to change; 'reaction' implies stronger opposition and desire to reverse

  • traditionalism

    focuses on preserving customs rather than opposing change actively

Antonyms

Grammar Patterns

reaction + against + noun

forces of + reaction

Usage Note

This sense is always uncountable ('reaction', not 'a reaction' or 'reactions'). The related noun 'reactionary' describes a person who holds such beliefs. Often carries a negative connotation.

Common Mistakes

The party represents a reaction
The party represents reaction' or 'The party is a reactionary force
this sense is uncountable.

7. [physics] a force of equal size that pushes backward when one thing pushes again

7.nounC1
Definition

[physics] a force of equal size that pushes backward when one thing pushes against another — described in Newton's third law, for instance the upward push a rocket gets when it forces hot gas downward.

Example

When Felipe pushes against the wall during a standing vertical jump, the wall's reaction force propels his body upward.

equal and opposite reaction — Newton's third law formula

A jet engine's thrust produces a reaction force that pushes the plane forward.

reaction force — physics collocation

Common collocations
  • equal and opposite reaction
  • reaction force
  • normal reaction
  • action and reaction
  • Newton's third law

Grammar Patterns

reaction + force

equal and opposite + reaction

action and + reaction

Usage Note

Often paired with 'action' as a fixed pair ('action and reaction'). In physics problems, 'reaction force' refers specifically to the force exerted by a surface against an object resting on it (normal reaction).

Common Mistakes

The action force equals the reaction' (missing 'opposite')
The action force equals the reaction force and points in the opposite direction
Newton's third law specifies both equal magnitude and opposite direction.

Word family

  • react
    verbB1
  • reactive
    adjectiveB2
  • reactor
    nounB2
  • overreact
    verbB2
  • overreaction
    nounB2
  • reactionary
    adjectiveC1
  • reactionary
    nounC1

Synonym discussion

Sense: noun/1

While all five words describe a return of some kind, they differ in scope and deliberateness. 'Reaction' is the broadest — it covers feelings, words, and actions, both voluntary and automatic. 'Response' is more deliberate and often implies that some thought or decision has taken place before answering. 'Reply' is narrower, used almost exclusively for verbal or written answers to a question or message. 'Feedback' is evaluative by nature — it is a reaction given specifically to help someone improve their performance or product. 'Backlash' describes a strong, angry negative reaction from a large group of people, usually against a policy, trend, or cultural change. 'Reflex' is purely automatic and physical, like pulling your hand away from a hot surface. For example: 'Her reaction to the news was to cry' (feeling + action); 'His response to the proposal was carefully worded' (deliberate); 'I received four replies to my email' (written); 'The company collected customer feedback' (evaluative); 'The new law faced a public backlash' (angry group reaction); 'Blinking is a natural reflex' (automatic).

response ↗reply ↗feedback ↗backlashreflex ↗

Common collocations

Nouns

Adverbs

Phrases

  • in reaction to
  • as a reaction to
  • a reaction against
  • action and reaction

Prepositions

Etymology

LatinFrenchEnglish

From Latin 're-' (back, again, in return) + 'agere' (to do, drive, act). The prefix-plus-verb formed 'reagere' in post-classical Latin, which passed into French as 'réaction'. English borrowed 'reaction' in the mid-1600s, first for physical and chemical processes (a force or change that 'acts back') and later for emotional and social meanings.