also: ツァイチエン
interjection
A Chinese loanword used as a casual farewell, similar to 'bye' or 'see you'. Not a standard Japanese word; used mainly in informal or playful contexts, often by people familiar with Chinese.
from Chinese zàijiàn
じゃあ、また明日。ツァイチェン！
Well then, see you tomorrow. Bye!
彼は中国語で「再見」と言って手を振った。
He said 'zàijiàn' in Chinese and waved his hand.
Borrowed from Mandarin Chinese 再見 (zàijiàn, 'goodbye'). The Japanese usage is a direct loan of the Chinese expression, not a native coinage.