Definition:Pi

Definition
The real number $\pi$ (pronounced pie) is an irrational number (see proof here) whose value is approximately $3.14159\ 26535\ 89793\ 23846\ 2643 \ldots$

Also see

 * Equivalence of Definitions of Pi

It is a common fallacy that the value of $\pi$ is dependent upon the geometry in which it is defined.


 * Sine and Cosine are Periodic on Reals/Pi, where $\pi$ is defined algebraically.