A diameter of a circle is a straight line through its center
which joins two points on its circumference, such as A and
B. Its length is twice that of the radius. Each diameter is
a line of symmetry for the circle and cuts it into two semi-circles.
The diameter is the longest chords of the circle.
more modern usage, the length of a diameter is also called
the diameter. In this sense one speaks of the diameter rather
than a diameter, because all diameters of a circle have the
same length, this being twice the radius.
Example of a Diameter: