Terminology
The two-round system is known as run-off voting in the United States, where the second round is known as a run-off election. Runoff voting is also sometimes used as a generic term to describe any system involving a number of rounds of voting, with eliminations after each round. By this broader definition the two-round system is not the only form of run-off voting, and others include the exhaustive ballot and instant run-off voting (also known as the alternative vote). However the subject of this article is the two round system.
In Canada, for example, candidates for party leadership, when there are more than two, use an exhaustive ballot system, often called a run-off voting system. It is like a run-off system, except the one candidate must win a simple majority (over half). Candidates with the fewest votes or candidates who want to move their support to other candidates may also move to remove themselves from the next vote.
Read more about this topic: Two-round System