nominee vs write-in

nominee and write-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
nominee	Yes	No	No	No
write-in	Yes	No	No	No

As nouns, write-in is a hyponym of nominee; that is, write-in is a word with a more specific, narrower meaning than nominee:

nominee: a politician who is running for public office
write-in: a candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters

nominee (noun)	write-in (noun)
a politician who is running for public office	a vote cast by writing in the name of a candidate who is not listed on the ballot
	a candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters