Pi is a mathematical constant. It's value is approximaly 3,14.
It is the ratio between a circle's circumference to it's diameter:
$$Pi = \frac{C}{d}$$
Pi is an irrational number, that is, it cannot be represented as a ratio of integers. This also implies that pi has infinitely many decimals, and isn't a repeating decimal. In fact a better approximaton for pi is: 3.141592653589793...