A closed primary is an electoral system in which only registered members of a particular political party are eligible to vote in that party's primary election. This means that only registered Democrats can vote in a Democratic primary, and only registered Republicans can vote in a Republican primary. The purpose of a closed primary is to ensure that only members of a particular party are able to select that party's nominee for a general election.
In a closed primary system, the rules for registering to vote may vary depending on the state or municipality. In some cases, voters must register with a specific party in order to vote in that party's primary. In other cases, voters may be able to register as "unaffiliated" or "independent" and choose which party's primary to vote in.
Closed primary system may also have different rules for voter registration cut-off dates, and for voter eligibility (like age, citizenship).
Closed primaries have the advantage of ensuring that only committed members of a party are able to select that party's nominee. This can help to prevent candidates who do not align with the party's platform or values from winning the nomination. However, closed primaries can also limit voter choice, as those who are not registered with a particular party are not able to participate in that party's primary.