suitability vs fitness

suitability and fitness both are nouns.

Word	Noun	Adjective	Verb	Adverb
suitability	Yes	No	No	No
fitness	Yes	No	No	No

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

suitability: the quality of having the properties that are right for a specific purpose
fitness: the quality of being suitable

Other hyponyms of suitability include appropriateness, fittingness, convenience.

suitability (noun)	fitness (noun)
the quality of having the properties that are right for a specific purpose	the quality of being suitable
	the quality of being qualified
	good physical condition; being in shape or in condition