noun
inscribed circle; incircle
Geometry term for a circle that is tangent to all sides of a polygon, especially a triangle.
三角形の内接円の半径を求めなさい。
Find the radius of the incircle of the triangle.
外接円 is a circumscribed circle (circumcircle) that passes through all vertices of a polygon, while 内接円 is inscribed inside and tangent to each side.
Compound of 内接 (inscribed, from 内 'inside' and 接 'touch') and 円 (circle).