incidental vs parenthetic

incidental is a noun but parenthetic is not a noun.

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

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

incidental (adjective)	parenthetic (adjective)
(sometimes followed by `to') minor or casual or subordinate in significance or nature or occurring as a chance concomitant or consequence	qualifying or explaining; placed or as if placed in parentheses
not of prime or central importance
occurring with or following as a consequence