A point which is a member of the set closure of a given set 
and the set closure of its complement set. If 
is a subset of 
, then a point 
is a boundary point of 
if every neighborhood of
contains at least one point in 
and at least one point not in 
.
Boundary Point
See also
BoundaryExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Boundary Point." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/BoundaryPoint.html