When you get to a very low-level, I think one way to look at it would be to say that the most simple form of message passing is simply invoking functions with arguments which contain message data.

Most people probably go straight into the multi-node pub/sub universe when "messaging" starts being thrown around, but it could just as well be direct method invocation with objects in the single node/process case.

The nuance and complexity seems to come from where those messages are stored, and how access to them (or knowledge of their availability).