upon · prep
upon
B2 on
Dictionary examples:
Please report to the reception desk upon arrival. (42.6)
Upon her head she wore a black velvet hat. (50.9)
once upon a time
B1 used at the beginning of a story to mean a long time ago
Dictionary examples:
Once upon a time, a lazy student invented a homework-doing robot. (0.0)
Once upon a time, a forgetful king lost his entire kingdom. (0.0)
Once upon a time, a teacher gave homework to her students' pets. (0.0)
Once upon a time there was a beautiful princess. (27.6)