1. Ballot used for absentee voting.
Source: Election Terminology Glossary - Draft, National Institute of Standards and Technology (NIST), https://pages.nist.gov/ElectionGlossary/
2. Ballot cast by a voter unable to vote in person at his or her polling place on Election Day.
Source: Glossary of terms database, U.S. Election Assistance Commission, https://www.eac.gov/glossary/
3. Historically, absentee ballots have been, simply, ballots cast by voters who are not able to vote in regular polling places due to travel or to some circumstance that makes it difficult for them to come to the polls. This definition is still true but incomplete. A number of states have adopted no-excuse absentee balloting, which allows any voter to vote by absentee ballot. In some states with no-excuse absentee voting, so-called “early voting” is really just the period during which voters can request and submit an absentee ballot. Because most absentee ballots are mailed to the election office, voter turnout campaigns sometimes call no-excuse absentee balloting “vote by mail,” even if the state does not have a formal vote by mail system.
Source: COUNTING VOTES 2012: A State by State Look at Voting Technology Preparedness, Verified Voting Foundation, Rutgers School of Law - Newark Constitutional Litigation Clinic & Common Cause Education Fund, https://countingvotes.org/sites/default/files/CountingVotes2012_Final_August2012.pdf
4. First used by Union and Confederate soldiers during the Civil War, an absentee ballot is issued, usually by mail, to a voter in response to a request for that ballot, under the assumption that the primary voting place is an in-person polling facility at which the voter cannot (or prefers not to) vote, hence they will be “absent” from the traditional polling place. In eighteen states, most voters are still required to provide a specific, legally valid excuse to qualify for such a ballot. As the percentage of voters in the US using mailed ballots to vote has increased (about 27% in 2018), regardless of their presence or absence on Election Day, the term “absentee ballot” has become less applicable.
Source: Vote at Home: Policy and Research Guide, National Vote at Home Institute, https://www.voteathome.org/wp-content/uploads/2019/03/VAH-Policy-and-Research-Guide.pdf