Tangent plane of a sphere

…When a line or a plane shares a single point with a sphere, they are said to be tangent to the sphere, and the shared point is called a tangent point. In this case, the line is called a tangent to the sphere, and the plane is called a tangent plane to the sphere. The tangent line or tangent plane is perpendicular to the radius that passes through the tangent point (Figure 2). …

*Some of the terminology explanations that refer to the "tangent plane of a sphere" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…直線または平面が球面とただ1点を共有するとき，それらは球面に接するといい，共有点を接点という。この場合，直線を球の接線，平面を球の接平面という。接線や接平面は接点を通る半径に垂直である(図2)。…

※「球の接平面」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