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