radius diameter and circumference
The distance around a circle is called the circumference. The distance across a circle through the center is called the diameter. pi is the ratio of the circumference of a circle to the diameter. Thus, for any circle, if you divide the circumference by the diameter, you get a value close to pi.
The radius of a circle is the distance from the center of a circle to any point on the circle. If you place two radii end-to-end in a circle, you would have the same length as one diameter. Thus, the diameter of a circle is twice as long as the radius.
The diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle.
Circumference is the linear distance around the edge of a closed curve or circular object.