Definition of once adverb from the Oxford Advanced American Dictionary

      

    once

     adverb
    adverb
    NAmE//wʌns//
     
     
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  1. 1 on one occasion only; one time I've only been there once. He washes his car once a week. She only sees her parents once every six months.
  2. 2 at some time in the past I once met your mother. He once lived in Alaska. This book was famous once, but nobody reads it today.
  3. 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.
  4. Idioms
      all at once
       
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    1. 1suddenly All at once she lost her temper.
    2. 2all together; at the same time I can't do everything all at once—you'll have to be patient.
    1. 1immediately; without delay Come here at once!
    2. 2at the same time Don't all speak at once! I can't do two things at once.
    (just) for once,just this once(informal)
     
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    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?
    going once, going twice, sold
     
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    = gone
    once again,once more
     
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     one more time; another time Once again the plane was late. Let me hear it just once more.
    once a…, always a…
     
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    used to say that someone cannot change Once an actor, always an actor.
    once and for all
     
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    now and for the last time; finally or completely We need to settle this once and for all.
    once bitten, twice shy(saying)
     
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    after an unpleasant experience you are careful to avoid something similar
    once in a blue moon(informal)
     
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    very rarely
    (every) once in a while
     
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    occasionally
    once or twice
     
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    a few times I don't know her well, I've only met her once or twice.
    once too often
     
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    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 upon a time
     
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    used, especially at the beginning of stories, to mean “a long time in the past” Once upon a time there was a beautiful princess.
See the Oxford Advanced Learner's Dictionary entry: once