Real Dictionary

write in
phrasal verb: If you write in to an organization, you send them a letter.
phrasal verb: In the United States, if someone who is voting in an election writes in a person whose name is not on the list of candidates, they write that person's name on the voting paper and vote for him or her.

write in in American English
to vote for (a candidate not listed on the ballot) by writing his or her name on the ballot
to vote for (someone not officially on a ballot) by inserting that person's name on the ballot

write in in British English
to insert in (a document, form, etc) in writing
noun: the act of voting for a person by writing his or her name on a ballot

write-in
In the US, a write-in is a vote that you make by writing the candidate's name on the ballot paper.

write-in in American English
a vote for some person whose name is not on the ballot, made by inserting that person's name
of or relating to a write-in
adjective: of, pertaining to, or for such a candidate or vote or a ballot so marked
noun: a candidate or vote for a candidate not listed on the printed ballot but written onto it by the voter

write-in in British English
noun: a vote cast for an unlisted candidate by writing his or her name on a ballot paper