00 What is π?

https://en.wikipedia.org/wiki/Pi

The number π, also Ludolph’s number, Ludolf’s number or Archimedes’ constant, is a mathematical constant defined as the ratio of the circumference of a circle to its diameter. This ratio is independent of the size of the circle.

The circle number starts with π = 3.141 5926 … , where only three digits are used in practical calculations.