A caucus is a gathering of like-minded individuals either to discuss and advance certain goals (if they meet regularly) or to elect a political candidate as part of the primary system (if used in the context of a broader election).
In the electoral system of the United States, each political party selects a nominee for office through a primary, and a caucus is a type of primary for selecting a nominee. A caucus consists of members of the party traveling to a central location to hear speeches, consider the issues, and then cast their votes to select the nominee of their party. Because of the significant effort required, a caucus usually results in a more knowledgeable, devoted and smaller number of voters than in broader primary systems that simply consist of voters stopping by a local polling booth.
The term "caucus" was one of John Adams' favorite terms. It is derived is from the Algonquian Indian language, having a general meaning of "to meet together." In Congress it means an informal organization of Members of the House or the Senate, or both, that exists to discuss issues of mutual concern and possibly to perform legislative research and policy planning for its members. There are regional, political or ideological, ethnic, and economic-based caucuses.
-  US Senate Reference