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A representative disk is three- dimensional volume element of a solid of revolution. The element is created by rotating a line segment (of length "w") around some axis (located "r" units away); such that, a cylindrical volume, of π∫r²w units, is enclosed.

A disk is said to be closed or open according to whether the region does or does not include its boundary. A ball is a disk in a space with more than two dimensions. See ball (mathematics). In particular, in a two dimensional Euclidean space, an open (respectively closed) disk is a circular area without (resp. with) its boundary circle.

In topology, a openIn topology and related fields of mathematics, a set U is called open if, intuitively speaking, you can "wiggle" or "change" any point x in U by a small amount in any direction and still be inside U. In other words, if x is surrounded only by elements of disk and a closedIn topology and related branches of mathematics, a set is called closed if its complement is open. This implies that a closed set contains its own boundary. Intuitively, if you are outside the set, and you "wiggle" a little bit, you will still be outside disk in a metric spaceIn mathematics, a metric space is a set (or "space") where a distance between points is defined. History Maurice Frechet introduced metric spaces in his work Sur quelques points du calcul fonctionnel Rendic. Palermo 22(1906) 1-74. Formal definition Formal are synonymous with an open ball and closed ball.

See also:

• unit disk.
• annulusAn annulus (from Latin anulus "little ring") is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. The adjective form is annular (for example, an annular eclipse). In complex analysis an annulus ''ann(a; r, R in t.