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In geometry, a 'disk' (also spelt 'disc') is the region in a plane bounded by a circle.
A disk is said to be ''closed'' or ''open'' according to whether or not it contains the circle that constitutes its boundary. In Cartesian coordinates, the open disk of center $(a,\; b)$ and radius ''R'' is given by the formula
:
while the closed disk of the same center and radius is given by
:$overline\{\; D\; \}=\{(x,\; y)in\; \{mathbb\; R^2\}:\; (x-a)^2+(y-b)^2\; le\; R^2\}$
The area of a closed or open disk of radius ''R'' is π''R''² (see π).
The ''ball'' is the disk generalised to metric spaces. However, sometimes "disk" is used to mean "ball".
In theoretical physics a disk is a rigid body which is capable of participating in collisions in a two-dimensional gas. Usually the disk is considered rigid so that collisions are deemed elastic.

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See also

See also

★ Unit disk, a disk with radius one

★ annulus (mathematics)

★ Disk algebra