parenthetical vs incidental

parenthetical and incidental both are nouns.

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

parenthetical (noun)	incidental (noun)
an expression in parentheses	an item that is incidental
	(frequently plural) an expense not budgeted or not specified

parenthetical (adjective)	incidental (adjective)
qualifying or explaining; placed or as if placed in parentheses	(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