A chord of a circle is a straight line segment whose endpoints both lie on a circular arc. The infinite line extension of a chord is a secant line [Secant line], or just secant. More generally, a chord is a line segment joining two points on any curve, for instance, an ellipse.
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The red segment BX is a chord (as is the diameter segment AB).
Bibliography
“Chord (Geometry).” 2023. Wikipedia, January. https://en.wikipedia.org/w/index.php?title=Chord_(geometry)&oldid=1135674923.