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English definition of “once”

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once

adverb uk   /wʌns/ us  

once adverb (ONE TIME)

A2 one single time: I went sailing once, but I didn't like it. We have lunch together once a month.at once C1 at the same time: They all started talking at once.for once B2 used when something happens that does not usually happen: For once, the bus came on time.just this once used to say that you will only do or request something on this particular occasion: All right, I'll give you a lift - just this once.once again ( also once more) B1 again, as has happened before: Once again, racist attacks are increasing across Europe.once more B1 one more time: I'd like to visit the colleges once more before we leave. again, as has happened before: I'm pleased that Daniel's working with us once more.once or twice a few times: I've seen him once or twice in town.(every) once in a while B2 sometimes but not often: We meet for lunch once in a while.once and for all C2 completely and in a way that will finally solve a problem: Our intention is to destroy their offensive capability once and for all.once in a lifetime only likely to happen once in a person's life: An opportunity as good as this arises once in a lifetime. the once on a single occasion: I've only played rugby the once, and I never want to play it again.
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once adverb (PAST)

B1 in the past, but not now: This house once belonged to my grandfather. Computers are much cheaper nowadays than they once were. Once-thriving villages stand deserted and in ruins.
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once

conjunction uk   /wʌns/ us  
B2 as soon as, or from the moment when: Once I've found somewhere to live I'll send you my address. Remember that you won't be able to cancel the contract once you've signed.at once B1 immediately: I knew at once that I'd like it here.
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(Definition of once from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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