When something happens overnight, it occurs during the hours of one night. An overnight party can also be called a sleepover or a slumber party. (Pack your sleeping bag, but you probably won't use it.)

You can use overnight as an adverb or adjective literally meaning "during the night" — like an overnight airplane flight or a guest who stays overnight on your couch. You can also use it figuratively, to mean "quickly" or "instant." After you publish your first book, you may be an overnight success; and after an important election, the world may seem to change overnight.

Definitions of overnight

adv during or for the length of one night

“the fish marinates overnight

adv happening in a short time or with great speed

“these solutions cannot be found overnight!”

adj lasting, open, or operating through the whole night

“an overnight trip”
all-night, nightlong
primarily temporal sense; being or indicating a relatively great or greater than average duration or passage of time or a duration as specified

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