Pi can defined as the ratio between the circumference and the
diameter of a circle. (Strictly speaking this is only true for flat,
Euclidean geometry. In other geometries the ratio can have other
values)
Pi is an irrational number, that is it can never be represented
as the ratio of two integers. This also means that the digits of pi
will never repeat themselves. (The rational number 22/7 is an
approximation to pi - 3.142857(1)... as opposed to 3.141592(7)... )
Pi can be represented as an infinite series of rational numbers.
For example Gregory's Series for x=1 yields: