Really depends on what you mean with your question. $$\pi$$ is a greek letter. In mathematics it is, among other things, commonly used for a certain real number. This real number is traditionally defined to be the circumference of a circle with diameter 1, and this definition is still used at most schools as far as i know. In Univeristy one often uses less geometric definitons, for example let $$\pi$$ be the smallest positive real number such that $$\cos \pi = -1$$ with the standard definiton of $$\cos x = \lim_{n\rightarrow \infty}\sum\limits_{i=0}^n \frac{(-1)^ix^{2i}}{(2i)!}$$. If you need an appoximation of $$\pi$$ as a rational rumber, many people use $$3.141$$, for some purposes even using $$3$$ might be sufficiant. If you need a better approximation there should be various websites there you can look it up.
