cheese
It is called a area.
The perimeter
The line connecting the points is called a chord; the part of the perimeter of the circle enclosed is called an arc; the area between an arc and the chord is called a circular segment.
It is called its perimeter.
border
axis of rotation
out line
The perimeter is the length around the outside of a shape, or the total of all the lengths of its sides. A shape that consists only of a line is simple: it has two sides, both of which are exactly the same length (from one end to the other and back along the other side of the line). Just measure the line, then double it. That is the perimeter.
It is a tangent line
Yes, a single circular line called the Glasgow Subway.
In a right circular cone a line from the vertex to the center of the circular base is perpendicular to the base. In an oblique circular cone that same line will not be perpendicular.
If it is a circular mirror the line would be called a radius. If it is a rectangular or square mirror it would just be called a diagonal or straight line... nothing special