Is there any such thing as a non-nested hierarchy?
I think this question is more about the semantics of the english language than about cs theory. Unless this has a connection to some problem. I think generally speaking, hierarchy of things just means there is a well ordered relation of these things. And nested just means that the things are nested within each other. The relation is defined based on nesting. So the military ranks are a hierarchy, but not a nested one because the groups aren't nested within each other.