An introduction of pi

Pi is the ratio of the circumference of a circle to its diameter; it can be calculated as circumference divided by diameter, or circumference/diameter.

It is an irrational number which starts 3.14159265358979323846264338327050..., but it is usually approximated to 3.14 or 22/7, though using the [π] button on a calculator makes it easier to enter a fairly accurate value.