The takeaway:

– An absentee ballot is generally used in every state to refer to a ballot filled out by a voter who cannot, for various reasons, physically make it to a voting location on Election Day.
– 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.

In popular discussions, some people will use the terms absentee ballots and mail-in ballots to mean the same thing: voting by mail, regardless of why. However, many people will use absentee ballots specifically to refer to ballots that are mailed when a person can’t vote in person, and use the term mail-in ballots in the context of voting policies that enable all people to vote by mail.