The boundary of a set 
is the set of points belonging to both the set
closure of 
and the set closure of its complement
set. These points are called boundary points.
The boundary is sometimes called the frontier.
Boundary
See also
Boundary Complex, 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 Resource. https://mathworld.wolfram.com/Boundary.html