1. (向某一机构)写信，发信
If you write in to an organization, you send them a letter.

2. (美国选举中)在选票上加写(非候选人的姓名)
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.

3. see also: write-in

1. write to an organization

e.g. Many listeners wrote in after the controversial radio program

2. cast a vote by inserting a name that does not appear on the ballot

e.g. Many voters wrote in the names of strangers