Anatomy of a Circle

A circle is the set of all points the same distance from a fixed center. That distance is the radius. Drag the slider to change it.

The radius r is the distance from the center to any point on the circle.

The diameter d is a chord that passes through the center. It is twice the radius: $d = 2r$.

A chord is any segment with both endpoints on the circle. The diameter is the longest possible chord.