Final answer:
An open primary is a nominating election where any qualified voter can participate, regardless of party affiliation, contrasting with a closed primary, which restricts voting to registered party members.
Step-by-step explanation:
An open primary is a type of primary election in which any qualified voter can cast a ballot regardless of party affiliation. This means that a voter can decide on the day of the election which political party's primary they wish to participate in, although they can only choose one. It contrasts with a closed primary, where only registered party members may vote for their party's candidates. The rationale behind an open primary is to encourage broader voter participation, although it can lead to raiding, where voters from one party purposefully vote in another party's primary to influence the selection of a weaker candidate.
Some states provide a variation known as a semi-open primary, where voters do not have to make their party preference public before the voting booth but must request a specific party’s ballot at the time of voting. Contrasting with these, a caucus is a more public, deliberative process where party members gather to debate and vote for candidates.