Let 
be a set of simple polygonal obstacles in the plane,
then the nodes of the visibility graph of 
are just the vertices of 
, and there is an edge (called a visibility edge) between vertices
and 
if these vertices are mutually visible.
Visibility Graph
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References
de Berg, M.; van Kreveld, M.; Overmans, M.; and Schwarzkopf, O. "Visibility Graphs: Finding the Shortest Route." Ch. 15 in Computational Geometry: Algorithms and Applications, 2nd rev. ed. Berlin: Springer-Verlag, pp. 307-317, 2000.Referenced on Wolfram|Alpha
Visibility GraphCite this as:
Weisstein, Eric W. "Visibility Graph." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/VisibilityGraph.html