disk (mathematics)

Open disk, closed disk, and circle.

A disk in mathematics, roughly speaking, is the "filling" of a circle. A flat (two-dimensional) disk of radius r consists of all the points that are at a distance ≤ r (closed disk) or < r (open disk) from a fixed point in the plane. More generally, an n-dimensional disk of radius r is the set of all points at a distance ≤ r (closed) or < r (open) from a fixed point in Euclidean n-space. A disk is the two-dimensional analog of a ball.