reaction
Pluralreactions
- 1What someone does, says, or feels in answer to something that has happened.
His immediate reaction to the criticism was to defend himself.
Public reactions to the new policy ranged from praise to outrage.
UsageUsually a reaction to something (not
incorrect: reaction on): a reaction to the news, my first reaction was to laugh.Similarresponse
Note
It covers words, feelings, and actions alike: a sudden laugh is a reaction to a joke, while "What is your reaction?" asks for an opinion.
- 2A process in which one or more substances are changed into one or more new substances.
The reaction between hydrogen and oxygen produces water.
Chemists slowed the reaction by lowering the temperature.
UsageUsually a reaction between substances, which chemists call fast or slow.
Note
In chemistry the word names the change itself, not anyone's response to it. Substances combine, break apart or have their atoms rearranged, and something of a different composition or structure results. Mass and energy are conserved along the way. The textbook case is acid meeting base: the two neutralize each other and leave a salt and water.
- 3An instance of the body responding badly to a drug, food, or other substance, causing harm or discomfort.
The patient suffered an allergic reaction to the contrast dye used in her scan.
Nurses watched for any reaction during the first hour after the vaccine was given.
UsageThe cause follows to: a reaction to penicillin. Often an allergic reaction, and you have or suffer one.
Note
Said of the body, the word carries its harm with it: she had a reaction to the antibiotic needs no adjective to sound bad, whereas a reaction to news can be welcome or not.
- 4A person's capacity to move or act fast in physical response to something sudden; used in the plural.
Quick reactions saved the cyclist when a car door swung open right in front of her.
Years of table tennis had given him lightning reactions.
UsageUsually quick, fast or lightning reactions. His reactions are quick describes the person; his reaction was quick, one moment.
Similarreflexes
Note
Here the word works in the plural and names a faculty rather than an event: a goalkeeper or a driver has quick reactions, slow reactions, lightning reactions.
Less common meanings
- 5An icon or emoji added to a posted message to express how the user feels about it.
He tapped a thumbs-up reaction on the group message.
Within an hour the post had hundreds of reactions, mostly hearts and laughing faces, but only a few comments.
UsagePeople usually add, leave or tap a reaction on a post or message.
Note
A word from messaging apps and social platforms: a small icon such as a thumbs-up attached to a post or message in place of a written reply. Unlike the general response, it is a visible object that platforms count, so a post collects reactions the way it collects replies.
- 6The force of equal size and opposite direction that a body exerts in return whenever another force is applied to it.
As she presses down on the floor, the reaction of the floor pushes up on her feet with equal force.
A rifle recoils because of the reaction to the force that drives the bullet down the barrel.
Note
Mechanics never deals in a single force: every push comes paired as action and reaction. The two act on different bodies, so they do not cancel on either body. A rocket drives its exhaust gases backward, and the gases drive the rocket forward just as hard. Which force of the pair is called the action is arbitrary; the other is then its reaction.
Checked 5 Nov 2026 · 4 facts
- Forces occur in equal and opposite action–reaction pairs in Newtonian mechanics. www1.grc.nasa.gov
- They act on different bodies and do not cancel on either individual body. They do cancel as internal forces when summing forces for a system containing both bodies. openstax.org
- A rocket pushes exhaust backward while the exhaust pushes the rocket forward with equal force. imagine.gsfc.nasa.gov
- Either force can be designated the action, with the other designated its reaction. en.wikipedia.org
- 7Politics aimed at undoing recent changes and restoring an earlier order, or a time when those politics or ideas regain strength or prevail.
The decade after the uprising was a period of deep reaction.
Historians describe the era as one of political reaction against the earlier reforms.
UsageOften with against, naming the changes being opposed: reaction against the reforms, years of reaction.
Similarbacklash
Note
Politics uses the word for a whole current of opinion: the pull back toward an earlier social or political order, and the years when that pull prevails over progressive movements. Its adjective, reactionary, carries the same meaning.
Practice
Can you tell the meanings of reaction apart? Answer 3 quick questions from this page, or write your own sentence and AI will check it.
Origin
This word came through Middle English 'reaccion' from Latin 'reāctiō', which comes from the verb 'reagō', formed from 're-' ('again') plus 'agō' ('to act'); it is equivalent to 'react' plus '-ion'.
Checked 5 Nov 2026 · 4 facts
- English reaction came through Middle English reaccion. en.wiktionary.org
- Latin reāctiō derives from the verb reagō. en.wiktionary.org
- Latin reagō combines re- meaning again and agō meaning to act. en.wiktionary.org
- Reaction is equivalent in formation to react plus -ion. en.wiktionary.org