parenthetic vs incidental

parenthetic is not a noun while incidental is a noun.

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

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

parenthetic (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