I read an article on Franklin Veaux's blog recently that touched on this. The definition given was something along the lines of "a hierarchical relationship is a relationship in which at least one partner has more say over their partners' relationship than they do".
Let me find the blog and link to it. It was very interesting.
Here you go.
The definition actually was: A poly hierarchy exists when at least one person holds more power over a partner's other relationships than is held by the people within those relationships.