In this case, the melody is the one tied to the lyrics. It's the primary melody as Tom said, unless I add a lead guitar or other instrument to take over and do a different melody for a while. So far, this is only vocal, keyboard, bass and drum. So I'm dealing with the primary melody / rhythm (drum) on a very elemental basis.
I've been listening to the radio with this topic in mind and found a good example of a tight relationship between primary melody and rhythm: Ben E. King's "Stand By Me" Now I'm trying to find an example of a loose relationship.