The system by which certain state parties select their delegates to the national nominating convention. A caucus serves to elect delegates (who support specific candidates) to county and state conventions who then elect delegates to the national convention. In contrast, a primary directly elects convention delegates.
The origin of the word "caucus" is disputed. It has been suggested that it is either an Algonquin word meaning "to consult or to speak" or even a variation of "caulkers," which were revolutionary-era assemblies of shipwrights in Boston. While a caucus was once the system by which every state selected its delegates, they fell out of favor at the turn of the 20th century. Florida was the first state to organize a primary in 1901. State parties that still conduct caucuses tend to be smaller due to the grassroots nature of the process.
Each state party has the ability to set the rules of their caucuses. With Iowa's presidential caucus looming in less than three weeks, let's look at how the Iowa process works.
Despite what will be announced in Iowa the evening of February 1, the ultimate victor will not be known for months. The caucuses serve to elect a candidate's preferred delegates at the precinct level to a county convention, where they then select delegates to a state convention. It's those state delegates who finally select presidential delegates. For example, Rick Santorum was the initially-reported winner in the 2012 GOP Iowa caucus. By the time the presidential delegates were selected, however, third-place Ron Paul secured the vast majority of the state's 28 delegates.
Even the systems by which the Iowa parties conduct their caucuses are different. For the Democrats, candidates' supporters gather across Iowa's nearly 1,700 precincts and attempt to persuade undecideds to support their candidates. They then stand in groups indicating support for a particular candidate. If a candidate's group fails to secure a specific percentage, based on the number of delegates a precinct is awarded, the group is disbanded and must choose another candidate. This process continues until candidates reach a specific threshold (somewhere between 15-25 percent). Republicans follow a similar process in persuading their neighbors but vote via secret ballot and use no narrowing of the process.
While some candidates and commentators describe Iowa as a must-win state, the eventual nominee often is not the Iowa winner, at least for the GOP. Iowa has only picked the GOP nominee twice in the last six caucuses. Conversely, Iowa has selected the eventual Democratic nominee in four of the last six caucuses.
Iowa's "first-in-the-nation" status first rose to prominence in the 1972 Democratic nominating race, when the then-state party chairman moved the primary up to allow for additional time to print the necessary materials. The timing stuck and has since become a time-honored tradition in the state.