Pi (not pie) is a mathematical term that is infinitely long. Normally 3.14 or 22/7 are used to equate, since it can't actually be written out.
Rarin is correct, but didn't actually to anwser your question
Pi, is a number used that was created to deal with circle's and sphere's, due to their strange shape
Pi can be used in the formulas concerning the area and circumference(perimeter) of a circle