Betweenness is a centrality measure of a vertex within a graph. Vertices that occur on many shortest paths between other vertices have higher betweenness than those that do not.
Definition
For a graph G: = (V,E) with n vertices, the betweenness CB(v) for vertex v is:
Where σst(v) = 1 if the shortest path from s to t passes through v and 0 otherwise.
See also
References
Freeman, L. C. (1977) A set of measures of centrality based on betweenness.
Sociometry 40, 35--41.