Pi

Pi (the symbol is the Greek letter π) is:

The ratio of the Circumference
to the Diameter
of a Circle.

In other words, if you measure the circumference, and then divide by the diameter of the circle you get the number π

It is approximately equal to:

3.14159265358979323846…

The digits go on and on with no pattern. In fact, pi has been calculated to over one million decimal places and still there is no pattern.