Caucus
From Ballotpedia
Some states hold a caucus instead of a primary. This was how we originally did it back in the day, but most states hold primaries now. Generally, any voter registered with the party may attend. At the caucus, delegates are chosen to represent the state's interests at the national party convention. After discussion and debate, an informal vote is taken to determine which delegates should be chosen. Basically, in a caucus you are voting for a delegate who represents your choice rather than the actual candidate, as you would in a primary.[1]
