Editing Disk (mathematics) (section)

{{Short description|Plane figure, bounded by circle}}
{{Redirect|2-ball|the basketball event|2Ball}}
{{Other uses|Disc (disambiguation){{!}}Disc}}

[[File:Circle-withsegments.svg|thumb|right|Disk with {{legend-line|black solid 3px|[[circumference]] ''C''}}
{{legend-line|blue solid 2px|diameter ''D''}}
{{legend-line|red solid 2px|radius ''R''}}
{{legend-line|green solid 2px|center or origin ''O''}}]]

In [[geometry]], a '''disk''' ([[Spelling of disc|also spelled]] '''disc''')<ref name="odm">{{cite book |title=The Concise Oxford Dictionary of Mathematics |first1=Christopher |last1=Clapham |first2=James |last2=Nicholson |publisher=Oxford University Press |year=2014 |isbn=9780199679591 |url=https://books.google.com/books?id=c69GBAAAQBAJ&pg=PA138 |page=138}}</ref> is the region in a [[plane (geometry)|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.<ref>{{cite book |title=Intuitive Concepts in Elementary Topology |series=Dover Books on Mathematics |first=B. H. |last=Arnold |publisher=Courier Dover Publications |year=2013 |isbn=9780486275765 |page=58 |url=https://books.google.com/books?id=TsbDAgAAQBAJ&pg=PA58}}</ref>

For a radius <math>r</math>, an open disk is usually denoted as <math>D_r</math>, and a closed disk is <math>\overline{D_r}</math>. However in the field of [[topology]] the closed disk is usually denoted as <math>D^2</math>, while the open disk is <math>\operatorname{int} D^2</math>.