1 B1 neutral
To begin a journey or trip.
"They set off early to avoid the rush-hour traffic on the motorway."
inseparable
set off
B1 neutral mixed transitive/intransitive
To begin a journey, to cause something to activate or explode, or to cause a reaction in someone or something.
In plain English
To start a trip, to make something like an alarm or firework go off, or to cause a strong reaction.
What does "set off" mean?
4 meanings, ordered from most common to least. Color-coded by CEFR level.
2 B1 neutral
To cause something such as an alarm, explosion, or fireworks to activate.
"The movement in the corridor set off the motion-sensor alarm."
separable
3 B2 idiomatic informal
To cause someone to start laughing, crying, or reacting strongly.
"One mention of her ex-boyfriend's name was enough to set her off."
separable
4 B2 idiomatic neutral
To make something look more attractive by contrast (often used with colours or design).
"The deep blue frame really sets off the watercolour painting."
separable
Usage tip
One of the most versatile 'set' phrasal verbs. The journey sense is intransitive; the activation and reaction senses are transitive. Also used in phrases like 'set off a chain reaction'.
Words that pair with "set off"
Natural word combinations native speakers use most often.
alarm bomb fireworks reaction laugh journey
How to conjugate "set off"
The five tense forms you'll use most often.
Base
set off
I/you/we/they
3rd person
sets off
he/she/it
Past simple
set off
yesterday
Past participle
set off
have + pp
-ing form
setting off
continuous
Hear "set off" in the wild
Listen to native speakers using "set off" in real YouTube videos — click a clip to watch it on Looplines.
Other ways to say "set off"
Swap in when you want variety — tap a linked one to explore it.
activate depart detonate leave provoke trigger
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