vertex in a degeneracy ordering of a undirected graph
See http://en.wikipedia.org/wiki/Degeneracy_(graph_theory)

Essentially, a degeneracy ordering is an ordering on the vertices that you get from repeatedly removing a vertex of minimum degree in the remaining subgraph. The degeneracy of the graph is the smallest number d such that every vertex has at most d neighbors that are later than it in the ordering. Among all possible orderings, the degeneracy orderings minimize this number.