Pi, or $\pi$, is commonly used in the context of circles in geometry.
In particular, the circumference $C$ of a circle that has a diameter $d$ is
$C = \pi d$.
Also, the area $A$ of a circle with radius $r$ is
$ A = \pi r^2$.
Note that $2r = d$.
This is how $\pi$ is defined. It is a little weird that $\pi$ is defined in the context of two equations, but that is how we understand what $\pi$ is. Interestingly, $\pi$ is an irrational number, meaning it cannot be written as a ratio of two integers. An approximation of $\pi$ is: