1 adv If something happens once, it happens one time only.
ADV with v
I met Wilma once, briefly..., Since that evening I haven't once slept through the night..., Mary had only been to Manchester once before.
Once is also a pronoun., pron the/this PRON
`Have they been to visit you yet?'<emdash10001`Just the once, yeah.'..., Listen to us, if only this once.
2 adv You use once with `a' and words like `day', `week', and `month' to indicate that something happens regularly, one time in each day, week, or month.
ADV a n
Lung cells die and are replaced about once a week..., We arranged a special social event once a year to which we invited our major customers.
3 adv If something was once true, it was true at some time in the past, but is no longer true.
ADV with v, ADV with be, ADV with group/cl
The culture minister once ran a theatre..., I lived there once myself, before I got married..., The house where she lives was once the village post office..., My memory isn't as good as it once was.
4 adv If someone once did something, they did it at some time in the past.
ADV with v
I once went camping at Lake Darling with a friend..., We once walked across London at two in the morning..., Diana had taken that path once.
5 conj If something happens once another thing has happened, it happens immediately afterwards.
The decision had taken about 10 seconds once he'd read a market research study..., Once customers come to rely on these systems they almost never take their business elsewhere...
6 If something happens all at once, it happens suddenly, often when you are not expecting it to happen.
all at once phrase PHR with cl
(=all of a sudden)
All at once there was someone knocking on the door.
7 If you do something at once, you do it immediately.
at once phrase PHR with v
I have to go, I really must, at once..., Remove from the heat, add the parsley, toss and serve at once..., The audience at once greeted him warmly.
8 If a number of different things happen at once or all at once, they all happen at the same time.
at once/all at once phrase PHR after v, PHR adj/n and adj/n
You can't be doing two things at once..., No bank could ever pay off its creditors if they all demanded their money at once...
9 For once is used to emphasize that something happens on this particular occasion, especially if it has never happened before, and may never happen again.
for once phrase PHR with cl (emphasis)
For once, dad is not complaining..., His smile, for once, was genuine.
10 If something happens once again or once more, it happens again.
once again/once more phrase PHR with v, PHR with cl
Amy picked up the hairbrush and smoothed her hair once more..., Once again an official inquiry has spoken of weak management and ill-trained workers.
11 If something happens once and for all, it happens completely or finally.
once and for all phrase PHR with v (emphasis)
We have to resolve this matter once and for all..., If we act fast, we can once and for all prevent wild animals in Britain from suffering terrible cruelty.
12 If something happens once in a while, it happens sometimes, but not very often.
once in a while phrase PHR with cl
Earrings need to be taken out and cleaned once in a while.
13 If you have done something once or twice, you have done it a few times, but not very often.
once or twice phrase PHR with cl, PHR with v
I popped my head round the door once or twice..., Once or twice she had caught a flash of interest in William's eyes...
14 Once upon a time is used to indicate that something happened or existed a long time ago or in an imaginary world. It is often used at the beginning of children's stories.
once upon a time phrase PHR with cl
`Once upon a time,' he began, `there was a man who had everything.'..., Once upon a time, asking a woman if she has a job was quite a straightforward question.
once in a blue moon →
If you give something or someone the once-over, you quickly look at or examine them.
give something or someone the once-over phrase V inflects, PHR after v
She gave the apartment a once-over.