Usually grouped together with presidential primaries, a caucus [1], refers mostly to the method of voting. Most famous are the Iowa caucuses [2], where small groups of like-minded people — the caucus — assemble to air opinions and reach a consensus. In the case of Iowa democrats, that consensus is reached by forming groups of supporters for each candidate and public discussion. Republicans caucus by what they call a "straw vote," which mimics a more traditional ballot-style election. Whether a state elects delegates by caucus or primary and what the delegates do with those results at the convention also varies by state [3], but the outcome is a party's candidate, officially named at the convention.

Source [4]