A Plebiscite is defined as a vote of the people, or a direct vote of all members of an electorate such as an entire county or region. Often the vote is to express an opinion about for or against a proposal, and in many situations this is about a choice of ruler, government, or change in a constitution.
For example, a nationwide election where all citizens who are eligible to vote (known as an electorate, or enfranchised individuals) do so for their choice of President or other leader would be a plebiscite. Another word or synonym for this is referendum.