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 
.
