# Disk (mathematics)

## Plane figure, bounded by circle / From Wikipedia, the free encyclopedia

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In geometry, a **disk** (also spelled **disc**)[1] is the region in a plane bounded by a circle. A disk is said to be *closed* if it contains the circle that constitutes its boundary, and *open* if it does not.[2]

Plane figure, bounded by circle

For a radius, $r$, an open disk is usually denoted as $D_{r}$ and a closed disk is ${\overline {D_{r}}}$. However in the field of topology the closed disk is usually denoted as $D^{2}$ while the open disk is $\operatorname {Int} D^{2}$.