incidentally vs incidental

incidentally is not a noun while incidental is a noun.

Word	Noun	Adjective	Verb	Adverb
incidentally	No	No	No	Yes
incidental	Yes	Yes	No	No

incidentally (noun)	incidental (noun)
	an item that is incidental
	(frequently plural) an expense not budgeted or not specified

incidentally (adjective)	incidental (adjective)
	(sometimes followed by `to') minor or casual or subordinate in significance or nature or occurring as a chance concomitant or consequence
	not of prime or central importance
	occurring with or following as a consequence

incidentally (adverb)	incidental (adverb)
introducing a different topic
of a minor or subordinate nature