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