Hypergraph
 

In mathematics, a hypergraph is a generalization of a graph
in which an edge can join any number of vertices.
In contrast, in an ordinary graph, an edge connects exactly two vertices.

A directed hypergraph differs in that its hyperedges are not sets,
but an ordered pair of subsets of X, constituting the tail and
head of the hyperedge.

Hypergraphs can be viewed as incidence structures.

In particular, there is a bipartite "incidence graph" or "Levi graph"
corresponding to every hypergraph, and conversely,
most, but not all, bipartite graphs can be regarded as
incidence graphs of hypergraphs.