auf
interj. C2 Proficiency auf
interj. a word used to say goodbye, usually as part of a longer German phrase. You might hear it in movies or read it in books when someone is leaving.
interj. a German preposition meaning 'on' or 'until', used in English primarily within the loan-phrase 'auf Wiedersehen'. Borrowed as a formulaic part of a parting salutation.
He waved and said 'auf Wiedersehen' as he left the room.
The traveler offered a polite 'auf Wiedersehen' to the hotel staff before heading to the airport.
Usage
Almost exclusively used within the fixed expression 'auf Wiedersehen'.