v.1. To cast a vote by inserting some name not listed on a ballot: The members of the minority party wrote in their candidate's name on their ballots. Write me in for mayor in the next election!
2. To insert something in a text or document: I wrote in a disclaimer at the end of the article. You forgot to cite your sources for the paper, so I wrote them in for you.
3. To insert a character or scene into a narrative work: The editor insisted that the author write in a happy ending to the story. I revised the play and wrote some new characters in.
4. To communicate with some organization by mail: Please write in with a completed entry form.