An -dimensional disk (sometimes spelled "disc") of radius is the collection of points of distance (closed disk) or (open disk) from a fixed point in Euclidean -space. A disk is the shadow of a ball on a plane perpendicular to the ball-radiant point line.
The -disk for is called a ball, and the boundary of the -disk is a -hypersphere. The standard -disk, denoted (or ), has its center at the origin and has radius .
More things to try:
Weisstein, Eric W. "Disk." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Disk.html