ability vs aptitude

ability and aptitude both are nouns.

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

As nouns, aptitude is a hyponym of ability; that is, aptitude 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
aptitude: inherent ability

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