reaction

noun

re·​ac·​tion rē-ˈak-shən 

1

a

: the act or process or an instance of reacting

b

: resistance or opposition to a force, influence, or movement

especially : tendency toward a former and usually outmoded political or social order or policy

2

: a response to some treatment, situation, or stimulus

her stunned reaction to the news

also : such a response expressed verbally

critical reaction to the play

3

: bodily response to or activity aroused by a stimulus:

a

: an action induced by vital resistance to another action

especially : the response of tissues to a foreign substance (such as an antigen or infectious agent)

b

: depression or exhaustion due to excessive exertion or stimulation

c

: heightened activity and overaction succeeding depression or shock

d

: a mental or emotional disorder forming an individual's response to their life situation

4

: the force that a body subjected to the action of a force from another body exerts in the opposite direction

5

a(1)

: chemical transformation or change : the interaction of chemical entities

(2)

: the state resulting from such a reaction

b

: a process involving change in atomic nuclei

Synonyms of reaction

• response
• answer

Examples of reaction in a Sentence

Their reaction to the news was positive.

When I told him what happened, his immediate reaction was one of shock.

The announcement produced an angry reaction from most of the people at the meeting.

My first reaction when I met him was not to trust him.

There hasn't yet been much reaction to the announcement.

There has been strong reaction against the government's policies.

A good driver has quick reactions.

The speed of his reactions saved his life.

Recent Examples on the Web

Grandma Belle’s Tomato Basil Soup, produced by Lil’ Turtles, may contain undeclared milk, which poses the risk of a serious allergic reaction in people with a milk allergy or sensitivity. —Jenna Anderson, Health, 12 Dec. 2025

The swift reactions came in the aftermath of Justin Tawse Brigham, 44, being charged Thursday with statutory rape of a child, indecent liberties with a child and first-degree burglary. —Chase Jordan, Charlotte Observer, 12 Dec. 2025

Both negative earnings reactions to Oracle and Broadcom this week have presented an opportunity to create income as AI looks to capture the next leg up. —Jeff Kilburg, CNBC, 12 Dec. 2025

In March, he was briefly hospitalized following an adverse reaction to his routine treatment for cancer. —Kelsie Gibson, PEOPLE, 12 Dec. 2025

Word History

First Known Use

1585, in the meaning defined at sense 1a

Phrases Containing reaction

• alarm reaction
• chain reaction
• chemical reaction
• counter-reaction
• cross-reaction
• dark reaction
• Feulgen reaction
• Hill reaction
• in reaction to
• light reaction
• Maillard reaction
• mixed reaction
• nuclear reaction
• polymerase chain reaction
• reaction gif
• Wassermann reaction

Rhymes for reaction

• abstraction
• attraction
• compaction
• contraction
• detraction
• diffraction
• distraction
• exaction
• extraction
• impaction
• inaction
• infraction

Kids Definition

reaction

noun

re·​ac·​tion rē-ˈak-shən 

1

a

: the act or process or an instance of reacting

our reaction to the news

b

: an action or attitude opposing current political or social forces or ideas

2

: a bodily response to a stimulus

especially : the response of the body to a foreign substance (as a drug)

3

: the force that opposes the action of a force applied to one body by another body

4

a

: chemical transformation or change : the action between atoms or molecules to form one or more new substances

b

: a process involving change in atomic nuclei

Medical Definition

reaction

noun

re·​ac·​tion rē-ˈak-shən 

1

: the act or process or an instance of reacting

2

: bodily response to or activity aroused by a stimulus:

a

: an action induced by vital resistance to another action

especially : the response of tissues to a foreign substance (as an antigen or infectious agent)

b

: depression or exhaustion due to excessive exertion or stimulation

c

: abnormally heightened activity succeeding depression or shock

d

: a mental or emotional disorder forming an individual's response to his or her life situation

3

: the force that a body subjected to the action of a force from another body exerts in the opposite direction

4

a(1)

: chemical transformation or change : the interaction of chemical entities

(2)

: the state resulting from such a reaction

b

: a process involving change in atomic nuclei