1. 円の中心点を通り、円周上の2点をつなぐ直線の長さ(the length of a straight line passing through the center of a circle and connecting two points on the circumference)
2. 円の中心と円周上の2点(または球の中心と球面上の2点)を結んでいる直線(a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface))