Definition of one day
- One day, it'll happen. You'll see.
- People may one day be able to take vacations to the moon.
- I went to her house one day and had lunch with her.
- One day, we had a terrible argument.
: the time of light between one night and the next
What made you want to look up one day? Please tell us where you read or heard it (including the quote, if possible).