Noun write-in has 2 senses

• write-in(n = noun.person) write-in candidate - a candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters; Array
is a kind of campaigner, candidate, nominee
Derived form verb write in₁

• write-in(n = noun.act) Array - a vote cast by writing in the name of a candidate who is not listed on the ballot; Array
is a kind of ballot, balloting, vote, voting
Derived form verb write in₁