The set of points, known as boundary points, which are members of the set closure of a given set and the set closure of its complement set. The boundary is sometimes called the frontier.
Boundary
See also
Boundary Conditions, Boundary Map, Boundary Point, Boundary Set, Natural Boundary, SurgeryExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Boundary." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Boundary.html