Boundary (topology) - Wikipedia
https://en.wikipedia.org/wiki/Boundary_(topology)In topology and mathematics in general, the boundary of a subset S of a topological space X is the set of points which can be approached both from S and from the outside of S. More precisely, it is the set of points in the closure of not belonging to the interior of An element of the boundary of is called a boundary point of The term boundary operation refers to finding or taking the boundary o…