Which description best fits a chord?

A chord is defined as a segment whose endpoints lie on the circumference of a circle. This means that it is entirely contained within the circle and connects two points on the circle's edge. The significance of this definition is that any straight line segment connecting two points on a circle's periphery is categorized as a chord, regardless of its length, as long as it doesn't extend beyond the circle.

The other descriptions do not accurately represent what a chord is. A line that is always outside the circle refers to external lines that do not intersect the circle at all. A line that touches the circle at one point describes a tangent line, which is different from a chord that spans across the circle. Lastly, while a segment equal in length to the diameter is indeed a specific type of chord, not all chords are equal to the diameter; therefore, that description is too narrow. The best and most comprehensive definition in the context provided is that a chord is a segment whose endpoints are on the circle.