good ˈnight S3
used to say goodbye when you are leaving someone or they are leaving at night, or before going to sleep ⇒ good evening:
Good night. Sleep well.

REGISTER
In informal everyday English, people often just say Night or, especially to children, Night night:
Night night, darling. See you in the morning.

[TahlilGaran] Dictionary of Contemporary English ▲

good night
interj.
1. Used as a polite phrase when you leave someone at night.
"Good night!" said Bob as he left Dick's house after the party. "I'll see you in the morning."
Bill said good night to his parents and went upstairs to bed.
2. or good grief
■ Used to show surprise and often some fear or anger.
Mr. Johnson's eyes opened wide when he saw the fish his little boy had caught, and said, "Good night!"
Mother was angry and said to Mary, "Good grief! Haven't you started the dishes yet?"

[TahlilGaran] English Idioms Dictionary ▲