"a private meeting of members of a political party to plan action or to select
delegates for a nominating convention."
Whereas, a primary is:
"[an] election held to nominate a candidate for a particular party at a
forthcoming election for public office."
It goes on to say that a primary is more like an election and a caucus is more like a town hall meeting with lots of pretty speachifying and such. It sounds to me as though you have to be a registered party member and have an invitation to be involved in a caucus. I'm not sure, however, why Washington has both. I'm also not sure if the caucus has any bearing on nominating candidates in this state. I've already done enough political research for one day, so I'm not going to find out right now.