Area = Pi r2
Circumference = Pi d

What is a circle?

A circle is a set of points equidistant from a center point. The distance from the center point to any point on the circle is the radius. The diameter of the circle is a chord that goes through the center. Its length is twice the length of the radius. The distance around a circle is the circumference.

If you divide any circle's circumference by its diameter, you get Pi.
Pi is approximately 3.14 or 22/7.