Open disk

…To make a clear distinction, the circle defined first is called the circumference, and the figure that is the combination of the circumference and the plane part that it encloses is called a disk. The inside of the circle, that is, the figure that remains after removing the circumference from the disk, is called an open disk. The line segment OP that connects the center O of the circle to a point P on the circumference is called the radius, and when the line segment AB that connects two points A and B on the circumference passes through O, the line segment AB is called the diameter. …

*Some of the terminology explanations that mention "open disk" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…はっきりと区別するため，最初に定義した円を円周といい，円周とそれが囲む平面の部分を合わせた図形を円板という。なお，円の内部，すなわち円板から円周を除いた残りの図形を開円板という。円の中心Oと円周上の1点Pを結ぶ線分OPを半径といい，円周上の2点A,Bを結ぶ線分ABがOを通るとき，線分ABを直径という。…

※「開円板」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報