Meaning of “once” - Learner’s Dictionary

once

adverb uk us /wʌns/
ONE TIME

A2 one time:

It's only snowed once or twice this year.
I go swimming once a week (= one time every week).
NOT NOW

B1 in the past, but not now:

This house once belonged to my grandfather.
once again

B1 again:

Once again I'm left with all the washing up.
all at once

suddenly:

All at once he stood up and walked out of the room.
at once

B1 immediately:

I knew at once that I would like it here.

at the same time:

They all started talking at once.
once in a while

B2 sometimes but not often:

He plays tennis once in a while.
once and for all

If you do something once and for all, you do it now so that it does not have to be dealt with again:

Let's get to the bottom of this matter once and for all!
once more

B1 one more time:

If you say that once more, I'm going to leave.
for once

B2 used to mean that something is happening that does not usually happen:

For once, I think I have good news for him.
once upon a time

B1 used at the beginning of a children's story to mean that something happened a long time ago

→ See also once in a blue moon

(Definition of “once adverb” from the Cambridge Learner’s Dictionary © Cambridge University Press)