A2 one time: It only snowed once or twice this year. I go swimming once a week (= one time every week).
B1 in the past, but not now: This house once belonged to my grandfather.
at once B1 immediately: I knew at once that I would like it here.
at once › at the same time: They all started talking at once.
once more B1 one more time: If you say that once more, I’m going to leave.
once upon a time B1 used at the beginning of a children’s story to mean that something happened a long time ago
once again B1 again: I’ll explain it once again.
for once › used to mean that something is happening that does not usually happen: For once, I think I have good news for him.