这两天被这些词搞得有点迷糊，所以查了一下。总结如下：
A ballot （选票） is the physical form (or electronic voting machine equivalent) that a voter fills out; it lists the candidates, issues, and so on that a person votes on.
Absentee, here, refers to a person who, for various reasons, can't physically be present at a voting center/location on Election Day. 由于某种原因无法到选举现场（站）进行投票的人。
An absentee ballot is a ballot used to cast an absentee vote, which is submitted, usually by mail, by an absentee. 由于某种原因无法到选举现场（站）进行投票的人的选票 - 暂且叫“不到场选票”。
A mail-in ballot is used more broadly to refer to ballots sent through the mail, including in all-mail voting states and some forms of absentee voting. 通过邮寄方式递送的选票 - 暂且叫“邮寄选票”。
All absentee ballots are sent through the mail (or very occasionally fax), but not all ballots sent through the mail are absentee ballots. “不到场选票”基本都是“邮寄选票”，也偶尔有可能是通过传真递送”传真选票“。但不是所有的“邮寄选票”都是“不到场选票”。有些州，比如Washington, Oregon, Colorado, Utah, and Hawaii已经实行邮寄投票，大部分居民自动收到邮寄的选票。但是，在这些州有些居民在他们所注册的邮寄地址收不到选票，譬如临时在国外等原因，因而他们需要索要选票以仍然以该州居民投票，这样只有他们的选票才能被称为是“不到场选票”。
“不到场选票”和“邮寄选票”，一个是基于投票人（能不能到场投票的）状态，一个是基于传递的方式。不能到场呢，那就通常都要邮寄或传真。但邮寄并不是你一定不能到场。
原文：https://www.dictionary.com/e/absentee-ballot-vs-mail-in-ballot/
有理解不对的地方请指正！
