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.