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:

$3.1415926535897932384626433832795028841971693993751058209749445923$.