A two-dimensional geometric figure, a line, consisting of the set of all those points in a plane that are equally distant from a given point (center).
“The set of all points (x, y) such that (x − 1)2 + y2 = r2 is a circle of radius r around the point (1, 0).”
A two-dimensional geometric figure, a disk, consisting of the set of all those points of a plane at a distance less than or equal to a fixed distance (radius) from a given point.