Once means one time, or at a single point in the past. When your teacher says you can check your answer once, she means one time, not twice or three times. When your grandfather tells you he once caught a fish as big as his arm, he means it happened at some point in his past, though maybe not recently.

The word appears in many common phrases. Once upon a time is how fairy tales begin, pointing to some moment long ago. Once in a while means occasionally, like seeing a shooting star. If something happens all at once, it happens suddenly, in a single moment rather than gradually.

Once can also mean “as soon as” or “when,” as in “Once you finish your homework, you can play outside.” Here the word connects two events: finishing homework makes playing outside possible.

People sometimes use once to emphasize that something used to be true but isn't anymore: “She was once the fastest runner in school” suggests someone else is faster now. This usage carries a hint of nostalgia or change, showing how time transforms things.