ability vs bilingualism

ability and bilingualism both are nouns.

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

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

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

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