さようなら

Etymology
Shortening of earlier, itself a compound of 🇰🇲. Literally “if that's the way it is”.

First cited to roughly 1742 as a conjunction (literally, “if it's like that, then...”). The interjection usage is cited to 1788, and then the noun sense is cited to roughly 1915.

Interjection

 * 1)   a final expression of departure; farewell, so long, adieu, goodbye
 * 2)   a non-final expression of departure: goodbye, au revoir

Usage notes
This term has strong connotations of finality, so this is not used when departing a place such as one's home, unless one intends not to return in the foreseeable future.

Synonyms
Use of these terms depends on the situation.

Noun

 * 1)  a farewell, a goodbye