This refers to a vote cast by someone who is unable to go to the polling station.
The system is designed to increase voter turnout. In some countries, the voter is required to give a reason for not going to the polling station, before participating in an absentee ballot. In India, a postal ballot is available to only some citizens.
The Representation of the People Act, 1950 allows heads of states and those serving in the armed forces to vote through postal means. The Lok Sabha recently passed a Bill to allow proxy voting for NRIs.
However, domestic migrants and absentee voters in India cannot cast postal votes.