Definition of once adverb from the Oxford Advanced American Dictionary

Oxford3000

once

adverb
/wʌns/
 
 
1 on one occasion only; one timeI've only been there once.He washes his car once a week.She only sees her parents once every six months.2 at some time in the pastI once met your mother.He once lived in Alaska.This book was famous once, but nobody reads it today.3 used in negative sentences and questions, and after if to mean “ever” or “at all”He never once offered to help.If she once decides to do something, you can't change her mind.IDIOMS

all at once

1 suddenlyAll at once she lost her temper.2 all together; at the same timeI can't do everything all at once—you'll have to be patient.all at once

at once

1 immediately; without delayCome here at once!2 at the same timeDon't all speak at once!I can't do two things at once.at once

(just) for once

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just this once

(informal) on this occasion (which is in contrast to what happens usually)For once he arrived on time.Can't you be nice to each other just this once?for oncejust for once

going once, going twice, sold

= going, going, gone at gonegoing once, going twice, sold

once again

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once more

one more time; another time
Once again the plane was late.Let me hear it just once more.once again

once a…, always a…

used to say that someone cannot change
Once an actor, always an actor.once a, always a

once and for all

now and for the last time; finally or completely
We need to settle this once and for all.once and for all

once bitten, twice shy

(saying) after an unpleasant experience you are careful to avoid something similaronce bitten, twice shy

once in a blue moon

(informal) very rarelyonce in a blue moon

(every) once in a while

occasionally
once in a whileevery once in a while

once or twice

a few times
I don't know her well, I've only met her once or twice.once or twice

once too often

used to say that someone has done something wrong or stupid again, and this time they will suffer because of it
You've tried that trick once too often.once too often

once upon a time

used, especially at the beginning of stories, to mean “a long time in the past”
Once upon a time there was a beautiful princess.once upon a time