Good night is what you wish people before everyone goes to sleep. Say good night, good night then, a good night kiss.
It is a farewell rather than a greeting, unlike good morning, which is why saying good night on arrival sounds wrong. Written as one word it becomes the thing itself rather than the wish.
1phrasegood nightWhat you wish people before everyone goes to sleep.
“Mum kissed us good night and switched off the bedroom light.”
good night is a farewell.
Also accepted in games: goodnight
Probably a shortening of a longer wish that somebody may have a good night.
Probably a shortening of "May you have a good night."
Etymology from Wiktionary, CC BY-SA
Practised in News & Communication