A heterarchy is a system of organization where the elements of the organization are unranked (non-hierarchical) or where they possess the potential to be ranked a number of different ways.
A heterarchy may be parallel to a hierarchy, subsumed to a hierarchy, or it may contain hierarchies; the two kinds of structure are not mutually exclusive. In fact, each level in a hierarchical system is composed of a potentially heterarchical group which contains its constituent elements.
The concept of heterarchy was first employed in a modern context by Warren McCulloch in 1945. As Carole L. Crumley has summarised, "[h]e examined alternative cognitive structure(s), the collective organization of which he termed heterarchy. He demonstrated that the human brain, while reasonably orderly was not organized hierarchically. This understanding revolutionized the neural study of the brain and solved major problems in the fields of artificial intelligence and computer design."
e.g. How one can attach tags to mails in google mail. Since tags can contain subtags a page can be multiple hierarchies at once.
Mathematical Term: Directed acyclic graph (DAG)
Interesting to think of FedWiki in this way. Each author and Fork organised in different ways and patterns. -- Tim Klapdor