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. It follows from this (after a nontrivial proof) that its notion of π is an irrational number---a number which cannot be written as a ratio of two whole numbers.