Pi is the ratio between the radius of a circular object and its circumference. It is notable for being a irrational number, impossible to display exactly as a fraction and in decimal terms it goies on forever without notably repeating itself. The only difference between the mathematical pi and American Pi is that American Pi goes on forever but repeats itself constantly. You can tell the bits that repeat itself, they are the only bits anyone doing this song in karaoke know (except for maybe the bit about the Jester).
Pi, as a mathematical formula is also notable for being about the death of Euclid, which was known in Greek terms as “The Day The Numbers Died”. American Pi is about the death of Buddy Holly, also known as the day everyobdy rejoices, sung hosannas and then got a nasty look from me because SINGING IS NO WAY TO CELEBRATE.