The word "sub-tends" refers to the geometric property of a line segment that extends from one point on a curve to another point on the same curve while passing through a third point that is not on the curve. Synonyms for this concept include "crosses," "intersects," and "passes through." Other related terms include "bisects," which means the line segment divides the curve into two equal parts, and "chords," which refers to line segments that connect two points on a curve but do not necessarily pass through a third point. Understanding these geometric concepts is crucial for various fields, including mathematics, engineering, and physics.