Pi is an irrational number and all of its digits are completely random. It's used to measure the area or perimeter of a circle. You can also use it to measure the volume and surface of a 3D object with a circle in it. The history of pi goes all the way back to the ancient Egyptians or Babylonians. The Egyptian thought pi was 3.16045 and the Babylonians thought it was 3.15. Pi is really aproximately 3.141592653589793238462643383279... but I can't go on forever. The definition of pi is the ratio of a circles circumpherence compared to it's diameter. I love to learn about pi even though I am only 11 years old. I have memorized 220 digits of it and I think that is alot more than most adults know.