An 
-dimensional open disk of radius 
is the collection of points of distance
less than 
from a fixed point in Euclidean 
-space. Krantz (1999, p. 3) uses the symbol 
to denote the open disk, and 
to denote the unit open disk centered at the origin.
The open disk for 
is called an open interval, and the term open
ball is often used for 
.
