peripheral vs incidental

peripheral and incidental both are nouns.

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

peripheral (noun)	incidental (noun)
electronic equipment external to the circuit board that contains the CPU of a computer	an item that is incidental
	(frequently plural) an expense not budgeted or not specified

peripheral (adjective)	incidental (adjective)
on or near an edge or constituting an outer boundary; the outer area	(sometimes followed by `to') minor or casual or subordinate in significance or nature or occurring as a chance concomitant or consequence
related to the key issue but not of central importance	not of prime or central importance
	occurring with or following as a consequence