night

(Good) night. 

1. the appropriate departure phrase for leave-taking after dark. (This assumes that the speakers will not see one another until morning at the earliest. Night alone is familiar.) John: Bye, Alice. Alice: Night. See you tomorrow. Bill: Good night, Mary. Mary: Night, Bill.

2. the appropriate phrase for wishing someone a good night's sleep. Father: Good night, Bill. Bill: Night, Pop. Father: Good night. Mother: Good night.

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See:

a hen night a night owl a one-night stand a stag night all night long at night be like ships that pass in the night black as a skillet by day call it a night day and night

different as night and day far into the night fly-by-night I must say good night I was up all night with a sick friend in the dead of night make a [day etc.] of it make a night of it morning, noon, and night night night after night

night and day night on the town night owl night person one-night stand Saturday night special ships that pass in the night sing before breakfast, you'll cry before night Time to call it a night