1- The distance between any two vertices u and v, denoted d(u, v), is the length of a shortest u − v path, also called a u − v geodesic.
2- Suppose G is a (weighted) graph and S a set of vertices in G. Then the Steiner distance for S, denoted by d_G(S), is the smallest weight of a connected subgraph of G containing S. Such a subgraph is necessarily a tree, called a Steiner tree for S. The radius, diameter and average distance have a natural extension. For a given vertex v in a connected (weighted) graph G and integer k (2 ≤ k ≤ n), the k-eccentricity of v, denoted by e_k(v), is the maximum Steiner distance among all k-sets of vertices in G that contain v. The k-radius, rad_k(G), of G is the minimum k-eccentricity of the vertices of G, and the k-diameter, diam_k(G), of G is the maximum k-eccentricity