An n-dimensional open disk of radius r is the collection of points of distance less than r from a fixed point in Euclidean n-space. Krantz uses the symbol D(x, r) to denote the open disk, and D = D(0, 1) to denote the unit open disk centered at the origin. The open disk for n = 1 is called an open interval, and the term open ball is often used for n>=3.

closed disk | disk | open ball | open interval | open set | perforation