bilingualism vs ability

bilingualism and ability both are nouns.

Word	Noun	Adjective	Verb	Adverb
bilingualism	Yes	No	No	No
ability	Yes	No	No	No

As nouns, ability is a hypernym of bilingualism; that is, ability is a word with a broader meaning than bilingualism:

bilingualism: the ability to speak two languages colloquially
ability: possession of the qualities (especially mental qualities) required to do something or get something done

Other hypernyms of bilingualism include power.

bilingualism (noun)	ability (noun)
the ability to speak two languages colloquially	the quality of being able to perform; a quality that permits or facilitates achievement or accomplishment
	possession of the qualities (especially mental qualities) required to do something or get something done