farewell vs valediction

farewell and valediction both are nouns.

Word	Noun	Adjective	Verb	Adverb
farewell	Yes	No	No	No
valediction	Yes	No	No	No

As nouns, valediction is a hyponym of farewell; that is, valediction is a word with a more specific, narrower meaning than farewell:

farewell (noun)	valediction (noun)
the act of departing politely	the act of saying farewell
an acknowledgment or expression of goodwill at parting	a farewell oration (especially one delivered during graduation exercises by an outstanding member of a graduating class)