In the US system of elections a caucus is when a group of people come together and agree upon who or what they're going to support. As I understand it, the word "caucus" emerged from a meeting long, long ago between men engaged in caulking the hulls of wooden ships. It was somewhere in the Irish Town of Boston Massachusetts. There's also the idea that caucus is Latin for drinking vessel.
The point is that at a caucus a person has to stand up in front of others and
come to some conclusion about his position on a subject. This means there's
plenty of room for peer pressure, bully, Stockholm Syndrome and people never
speaking to each other again. Primaries on the other hand, are secret ballots, a
person doesn't have to stand up to be counted.