A boundary is that which is the extremity of something.
Let \(c\) be a figure of a plane \(p\)1. A point \(A\) is called a boundary point of \(c\), if every2 circle with the centre at \(A\) and a radius \(r > 0\) has points in common with the figure \(c\) and with \(p\setminus c\) (i.e. the everything in the plane \(p\) except the figure \(c\):
The set of all boundary points of \(c\) is called the boundary of the figure \(c\) and denoted by \(\delta c\).
Definitions: 1 2 3
Propositions: 4 5