An attempt at a complete list. This is one overworked letter. :)

  1. The 16th letter of the Greek alphabet.
  2. Ancient Greek term for the circumference of a circle.
  3. Modern term, as N-wing mentioned, for the irrational ratio of a circle's diameter to its circumference. This ratio pops up in many other places too.
  4. N-wing states population percentage in statistics. I'll take his word for it.
  5. Product series when capitalized. As in, Π(1<x<5) of (x-1), x in Z =(2-1)(3-1)(4-1)
  6. π(x) is the number of primes below a number x - for example, π(100) = 25 (this, btw, approaches x/ln(x))

Given its general popularity as a concept meaning product, I'm sure there are other uses in various sciences. Will keep on hunting. Just had to add #6, my personal area of obsession.