act
act — verb
1. to do things in a certain way, showing a particular kind of behaviour.
to do things in a certain way, showing a particular kind of behaviour.
The children acted well while their grandparents were visiting for dinner.
act + adverb for behaviour
At the interview, Mina acted calm even though her hands were shaking.
act + adjective
Why are you acting so strange around your new piano teacher?
On stage, Leo acted like a king and bowed to nobody.
act like + noun
- act well
- act calm
- act strange
- act like a king
- behave
the closest general synonym
- conduct yourself
more formal and often used in rules or advice
Grammar Patterns
act + adjective
act like + noun
Usage Note
Often followed by an adjective or by 'like + noun' to show the kind of behaviour. Distinguish from sense 5, which usually suggests annoying or foolish behaviour.
Common Mistakes
2. to take steps instead of waiting, usually to achieve something or handle a probl
to take steps instead of waiting, usually to achieve something or handle a problem.
The town acted fast after the river rose above the bridge.
act + adverb
Seeing smoke in the kitchen, Rosa acted to keep the children outside.
act to + verb
No one acted until the teacher saw the broken window.
After weeks of delay, the company finally acted on the safety report.
act on + report
- act fast
- act now
- act to protect
- act on a report
Grammar Patterns
act to + verb
act on + report / advice / information
Usage Note
This sense focuses on doing something practical. Common with adverbs like 'quickly' and with patterns such as 'act to...' or 'act on...'.
Common Mistakes
3. to start producing a result, especially in the body or on a material.
to start producing a result, especially in the body or on a material.
The cream acted within minutes and the baby's rash began to fade.
act within + time
The medicine acted slowly, so Mr. Lin stayed in bed all morning.
act + adverb with medicine subject
Once the heat acted on the ice, water ran across the table.
act on + thing affected
By midnight, the coffee had acted, and Nina felt fully awake.
- act quickly
- act slowly
- act on the skin
- act within minutes
- work
the most common everyday alternative
- take effect
often used for medicine, rules, and plans
- fail
to produce no result
Grammar Patterns
act on + body part / material
act within + time
Usage Note
The subject is often something that causes change, such as a drug, heat, pressure, or a policy. Do not confuse this with sense 6, where something acts as another thing.
Common Mistakes
4. to appear as a character before an audience or camera.
to appear as a character before an audience or camera.
Ken acted the doctor in our school play last Friday.
act + role noun
In the film, Sara acted opposite a famous singer from Seoul.
act opposite + performer
During college, Omar acted in three musicals and one TV advert.
act in + production
At rehearsal, Mei acted with such skill that everyone fell silent.
- act in a film
- act on stage
- act the part
- act opposite someone
Grammar Patterns
act in + play / film
act + role noun
Usage Note
Often followed by 'in' for the production, or by a role noun in more formal stage language. This is the only verb sense tied to theatre and film performance.
Common Mistakes
5. to make yourself look foolish, annoying, or unnatural, often to get attention.
to make yourself look foolish, annoying, or unnatural, often to get attention.
Stop acting silly and put your shoes back on.
act + adjective in commands
At lunch, Dylan acted like a boss and ordered everyone around.
act like + noun
Even at twelve, Nora still acts like a baby sometimes.
The new student acted cool, but nobody found him funny.
act + image adjective
- act silly
- act cool
- act like a baby
- act like a boss
- fool around
more playful and informal
- show off
stresses trying to get attention
Grammar Patterns
act silly / cool / superior
act like + noun
Usage Note
Common in spoken English when criticizing someone's manner. It often suggests that the behaviour is not natural or not suitable for the situation.
Common Mistakes
6. to do the job of something, or be used in its place.
to do the job of something, or be used in its place.
For one week, the spare room acted as a home office.
act as + role
In the emergency, Lena acted as team leader until noon.
act as + temporary job
A folded towel can act as a pillow on the train.
act as + substitute object
During repairs, the old bridge acted as the town's only road.
- act as leader
- act as a guide
- act as a bridge
- act as a pillow
- serve as
the closest neutral equivalent
- function as
slightly more formal and common in technical writing
Grammar Patterns
act as + role
act as + substitute object
Usage Note
Almost always followed by 'as + noun'. The subject may be a person, room, object, or system that fills a role for a time.
Common Mistakes
act for
phrasal verbTo represent someone or do something on their behalf.
The lawyer acted for the family during the court case.
Who will act for you while you are away?
act on
phrasal verbTo take action because of something such as advice or information.
The police acted on a tip and arrested the suspect.
You should act on your doctor's recommendations immediately.
act on behalf of
phrasal verbTo do something for someone else as their representative.
The agent acted on behalf of the singer to negotiate the contract.
I am acting on behalf of my grandmother to sell her house.
act out
phrasal verbTo express thoughts or feelings through behavior, often in a dramatic way.
The child acted out his anger by throwing toys.
In therapy, patients sometimes act out their fears.
act up
phrasal verbTo behave badly or not work properly.
My computer is acting up again; it keeps freezing.
The children acted up during the long car ride.
act upon
phrasal verbTo take action based on something such as advice or information.
The committee acted upon the report's recommendations.
Have you acted upon the feedback from your manager?
act/play the fool
idiomTo behave in a silly or stupid way to make others laugh, sometimes causing annoyance.
During the meeting, Tom played the fool and made everyone laugh.
Stop acting the fool and help me clean the kitchen.
be/act your age
idiomTo behave in a way that is appropriate for how old you are, not like a child.
Mia, please act your age and stop throwing tantrums.
Why can't you just be your age and handle this calmly?
To behave in an overly confident or bossy way that annoys others.
The new intern acted as if he owned the place on his first day.
She thinks she owns the place, telling everyone what to do.
act — noun
1. something a person does, especially one seen as good, bad, or important.
something a person does, especially one seen as good, bad, or important.
Giving the driver an umbrella was a kind act on that wet night.
kind act
The simple act of opening the window made the room feel safer.
the act of + -ing
Police treated the fire as a criminal act from the start.
criminal act
Her act of honesty saved the shop from a costly mistake.
act of + abstract noun
- a kind act
- a criminal act
- an act of honesty
- the act of opening
Grammar Patterns
an act of kindness / violence / honesty
the act of + -ing
Usage Note
Often used with 'of' to name the kind of deed: an act of kindness, an act of violence, the act of telling the truth.
Common Mistakes
2. behaviour put on to hide what you really feel or want.
behaviour put on to hide what you really feel or want.
Was Mia's laugh real, or was it only an act?
just an act
After the phone call, Ben dropped the act and sat quietly.
drop the act
The smile was an act to stop the children asking questions.
At dinner, her cheerful act fooled everyone except Aunt June.
cheerful act that hides truth
- just an act
- put on an act
- drop the act
- a cheerful act
Grammar Patterns
be just an act
drop the act
put on an act
Usage Note
Usually refers to fake behaviour rather than a single lie. Common with verbs like 'put on' and 'drop'.
3. one short item in an entertainment programme, or the person doing it.
one short item in an entertainment programme, or the person doing it.
The first act was a drummer from Kaohsiung with bright blue hair.
act = performer
After the clown act, a singer walked on with a red guitar.
clown act
Tonight's final act will perform high above the circus ring.
final act
Their comedy act lasted ten minutes and ended with falling pies.
comedy act
- opening act
- comedy act
- circus act
- final act
Grammar Patterns
a comedy / circus / opening act
the next act is ...
Usage Note
Common in concerts, circuses, and variety shows. It can mean the performance itself or the person or group performing it.
Common Mistakes
4. one main section of a play, opera, or ballet.
one main section of a play, opera, or ballet.
The lights went down again at the start of Act Two.
Act + number
In the final act, the lost brothers meet beside the sea.
in the final act
During the second act, someone in row five answered a phone.
The director cut one song from the first act of the opera.
first act of + show
- Act Two
- the first act
- the final act
- the second act
Grammar Patterns
Act One / Act Two
the first / second / final act
Usage Note
An act is larger than a scene. In printed scripts, it is often written with a capital letter and a number.
5. a written law that a legislature or parliament has formally approved.
a written law that a legislature or parliament has formally approved.
Congress passed an act to protect children working online.
pass an act
Under the new act, stores must label every imported egg.
under the Act
The judge explained how the act changed voting rules statewide.
A later act gave small farmers more time to repay loans.
later act
- pass an act
- under the Act
- a later act
- an act to protect
Grammar Patterns
pass an act
under the Act
an act to + verb
Usage Note
This is a legal sense and often appears with a capital letter in the full official title of a law.
Common Mistakes
Word family
Synonym discussion
Sense: verb/2
For verb/2, act is the broadest choice. It simply means doing something instead of waiting: a person, company, or government can act. Move is more informal and often suggests speed, as in 'We need to move fast.' React means acting because something has already happened, so it stresses response rather than initiative. Intervene is narrower and more formal: it means stepping into a situation in order to change it, especially a difficult one. If a school fixes a broken gate, it acts. If it fixes it after complaints, it reacts. If the principal steps in personally during a fight, the principal intervenes.
Common collocations
Nouns
- act of kindness
- act of violence
- opening act
Phrases
- act now
- put on an act
- under the Act
Etymology
Act came into English in the Middle English period through French and Latin. Its older Latin relatives, actus and actum, were linked to agere, a verb meaning 'do', 'drive', or 'set in motion'. Because those Latin words could refer both to movement and to a thing done, English act developed several connected meanings: doing something, having an effect, performing on stage, and a deed or formal law.