The contact graph of a packing of balls is the simple graph with one graph vertex for each ball and a graph edge between each pair of balls that touch. The ball interiors are pairwise disjoint, so contact occurs at their boundaries. For a nonempty finite packing, the average vertex degree is twice the number of contacts divided by the number of balls (Dostert et al. 2022).
Taking the supremum of this average over finite ball packings in 
, allowing unequal radii, defines the
average kissing number.
