A United States presidential nominating convention is a political convention held every four years in the United States by most of the political parties who will be fielding nominees in the upcoming U.S. presidential election. The formal purpose of such a convention is to select the party's nominee for President, as well as to adopt a statement of party principles and goals known as the platform and adopt the rules for the party's activities, including the presidential nominating process for the next election cycle.
In the modern U.S. presidential election process, voters participating in the presidential primaries are actually helping to select many of the delegates to these conventions, who then in turn are pledged to help a specific presidential candidate get nominated. Other delegates to these conventions include political party members who are seated automatically, and are called "unpledged delegates" because they can choose for themselves for whom they vote for.
Generally, usage of "presidential campaign nominating convention" refers to the two major parties' quadrennial events: the Democratic National Convention and the Republican National Convention. Some minor parties also select their nominees by convention, including the Green Party, the Socialist Party USA, the Libertarian Party, the Constitution Party, and the Reform Party USA.