Definition:Circle/Chord

Definition

A chord of a circle is a straight line segment whose endpoints are on the circumference of the circle.

In the diagram above, the lines $CD$ and $EF$ are both chords.

Thus a chord is a line segment joining the endpoints of an arc of the circle.

Note that under this definition, the diameter itself is in fact a chord.

In the words of Euclid:

A straight line is said to be fitted into a circle when its extremities are on the circumference of the circle.

(The Elements: Book $\text{IV}$: Definition $7$)

Sources

• 2008: Ian Stewart: Taming the Infinite ... (previous) ... (next): Chapter $5$: Eternal Triangles: The origins of trigonometry