Describes the diameter, a straight line connecting two points of a circle that includes the center of the circle as one of its points. The answer is pi or 3.14159265359, always remember that diameter in a circle is always one-half or pi. The diameter of a circle is any straight line that passes through the center and endpoints of a circle is lies within the circle. It is also described as the longest chord of the circle. Diameter is the length of the line whose passes through the center of the circle and touching the two end points of the circle. It is also divided into two part called the radius. The radius is called when you half the circle so the radius is half of the circle.