write-in (Noun) — A candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters.
write-in (Noun) — A vote cast by writing in the name of a candidate who is not listed on the ballot.
write-in (Verb) — Cast a vote by inserting a name that does not appear on the ballot. ex. "Many voters wrote in the names of strangers"
write-in (Verb) — Write to an organization. ex. "Many listeners wrote in after the controversial radio program"