A primary election is an election used either to narrow the field of candidates for a given elective office or to determine the nominees for political parties in advance of a general election. Primary elections can take several different forms. In a partisan primary, voters select a candidate to be a political party's nominee for a given office in the corresponding general election. Nonpartisan primaries are used to narrow the field of candidates for nonpartisan offices in advance of a general election. The terms of participation (e.g., whether only registered party members can vote in a party's primary) in primary elections can vary by jurisdiction, political party, and the office or offices up for election. The methods employed to determine the outcome of the primary (e.g., plurality systems, majority systems, top-two systems, etc.) can also vary by jurisdiction.
Source: Ballotpedia