Final answer:
A second-round election where voters choose between the top two candidates is known as a runoff election. It occurs when no candidate wins a majority in the first round. Variations include Instant Run-Off Voting and the top-two primary method.
Step-by-step explanation:
A "second-round" election in which voters choose between the top two candidates from the first round is called a runoff election. This type of election is necessitated when no candidate receives the majority, which is identified as more than 50 percent of the votes cast. It primarily occurs in systems with majoritarian voting schemes. The general election may not follow a primary or caucus in these cases.
In some states, a variation of runoff election is observed, known as Instant Run-Off Voting (IRV), where voters rank their preferences, potentially avoiding the need for an actual second round. However, if a second round is still needed, it involves only the top two candidates.
Another similar concept is the top-two primary method, which places all candidates against each other regardless of party affiliation, and the two highest vote-getters advance to the general election. This can result in two candidates from the same party competing in the election.