circumference

The distance around a circle, calculated by the formula C = d.

diameterThe length across a circle, passing through the center of the circle. A diameter is equal to the length of two radii.
pi

The ratio of a circle’s circumference to its diameter. Pi is denoted by the Greek letter . It is often approximated as 3.14 or.

radiusThe distance from the center of a circle to any point on the circle.