good-by vs farewell

good-by and farewell both are nouns.

Word	Noun	Adjective	Verb	Adverb
good-by	Yes	No	No	No
farewell	Yes	No	No	No

As nouns, farewell is a hypernym of good-by; that is, farewell is a word with a broader meaning than good-by:

Other hypernyms of good-by include word of farewell.

good-by (noun)	farewell (noun)
a farewell remark	the act of departing politely
	an acknowledgment or expression of goodwill at parting