incidental vs peripheral

incidental and peripheral both are nouns.

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

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

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