fortitude vs grit

fortitude and grit both are nouns.

Word	Noun	Adjective	Verb	Adverb
fortitude	Yes	No	No	No
grit	Yes	No	Yes	No

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

fortitude: strength of mind that enables one to endure adversity with courage
grit: fortitude and determination

Other hyponyms of fortitude include backbone, gumption, guts, moxie, sand.

fortitude (noun)	grit (noun)
strength of mind that enables one to endure adversity with courage	fortitude and determination
	a hard coarse-grained siliceous sandstone

fortitude (verb)	grit (verb)
	clench together
	cover with a grit