Pi

Pi, or π, is the ratio between the circumference and the diameter of a circle. The numerical value of π depends on context: which circle is under consideration, and how precise are one's measurements?

The traditional concept

Standard mathematics says that π is the ratio between the circumference and the diameter of a perfect circle.

Standard mathematics "measures" this ratio using the methods of calculus.

Standard mathematics' notion of π is an irrational number---a number which cannot be written as a ratio of two whole numbers.

Thus in standard mathematics,

π = 3.141592653589793238462643383279502884197169399375105 8209749445923078164062862089986280348253421170679...

where the "..." signifies that the digits go on forever.