A caucus is most generally defined as being a meeting of supporters or members of a political party or movement. The exact definition varies between different countries.

In the United States, a caucus is a meeting of local members of a political party or subgroup to nominate candidates, plan policy, etc., in the Congress of the United States or other similar representative organs of government. One of the best-known examples is the Congressional Black Caucus, a group of African-American members of Congress. Other examples include the caucuses used by some states to select presidential nominees, such as the Iowa caucuses.

In some Commonwealth nations, a caucus is a regular meeting of all Members of Parliament who belong to a political party. In a Westminster System, a party caucus can be quite powerful, as it has the ability to elect or dismiss the party's parliamentary leader. The caucus also determines some matters of policy, parliamentary tactics, and disciplinary measures against disobedient MPs. In some parties (such as the Australian Labor Party or the New Zealand Labour Party), caucus also has the ability to elect MPs to Cabinet when the party is in government.

In New Zealand and in the Australian Labor Party, the term "caucus" can be used to refer to the collective group of the MPs themselves, rather than merely the meeting of these MPs. Thus, the (Australian) Federal Parliamentary Labor Party is commonly called "the Labor Caucus." The word was introduced to Australia by King O'Malley, an American-born Labor member of the first federal Parliament in 1901, and presumably entered into New Zealand politics at a similar time. In New Zealand, the term is used by all political parties, but in Australia, it is restricted to the Labor Party. In the Liberal Party of Australia, and for all parties in the United Kingdom and the Republic of Ireland, the usual term is the parliamentary party.

The usage of caucus in Canada is similar to that of New Zealand; caucus refers to all members of a particular party elected to parliament or a provincial legislature. In Canada, these members elect among themselves a caucus chair who presides over their meetings and is an important figure when the party is in opposition and an important link between cabinet and the backbench when the party is in government.

The origin of the word "caucus" is debated, although it is generally agreed that it came into use in English in the United States. According to some sources, it comes from the Algonquin word for "counsel," cau´-cau-as´u, and was probably introduced into American political usage through the Democratic Party machine in New York known as Tammany Hall, which liked to use Native American terms. Other sources claim that it derived from Medieval Latin caucus, meaning "drinking vessel", and link it to the Boston Club . In the Finnish language, there is also a word, kokous, meaning an official meeting. The existence of this word in the Finnish language supports the theory that it has European origins. It is also known that many Finns moved to the United States, more specifically to the East Coast and the areas surrounding the Great Lakes.

Lewis Carroll satirized the behavior of some caucuses in chapter III of Alice's Adventures in Wonderland, with a confused caucus race at the end of which the Dodo said "EVERYBODY has won, and all must have prizes."