1. <character> The greek lower-case letter P.
2. <mathematics> The mathematical constant that is the circumference of a circle divided by its diameter. Pi is written as the greek character pi.
Some have suggested that pi is the wrong choice and a better constant to describe the geometry of circles would have been 2*pi, for which they have proposed the name tau. Most practising mathematicians think this is silly. The xkcd comic strip proposed (http://xkcd.com/1292/) a compromise between pi and tau, namely 1.5*pi or "pau".