Pi (π)

Draw a circle with a radius of 1.

The distance half way around the edge of the circle
will be 3,14159265... a number known as Pi

Or you could draw a circle with a diameter of 1.

Then the circumference (the distance all the way
around the edge of the circle) will be Pi

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

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