An absentee ballot is a vote cast by somebody in a place which is not that person's usual place of residence.
When a person eligible to vote is away from his hometown, he can vote at a different polling station. Some countries allow voters unable to attend the polling station to vote by mail.
Military service members can vote by absentee ballots as well, usually by mail. Every installation and military unit is required to appoint a voting assistance officer (VAO), a person responsible for providing information and for helping service members and their families to vote in the correct way.
