A primary election in which only voters that belong to a particular political party are permitted to vote, e.g. only registered Democrats can vote in a closed Democratic party primary election.

a primary election in which only those voters who have registered as belonging to a particular political party can vote. For example, if it is a Republican primary election, only those people who are registered Republicans can vote (since that election is to choose the Republican candidate who will eventually run for office in the general election).

A primary election system in which voters can only vote for candidates from the party in which they are registered.

A closed primary is a type of direct primary limited to registered party members, who must declare their party affiliation, generally in advance of the election, in order to vote in it. The closed primary serves to encourage party unity and prevent members of other parties from infiltrating and voting to nominate weak candidates. The opposite is a open primary, that is open to voters regardless of their party affiliation.