Succinctly, pi—which is written as the Greek letter for p, or π—is the ratio of the circumference of any circle to the diameter of that circle. Regardless of the circle's size, this ratio will always equal pi. In decimal form, the value of pi is approximately 3.14. But pi is an irrational number, meaning that its decimal form neither ends (like 1/4 = 0.25) nor becomes repetitive (like 1/6 = 0.166666...). (To only 18 decimal places, pi is 3.141592653589793238.) But it GOES ON...FOREVER...in fact, they test super computers by making them calculate Pi! (from: https://www.scientificamerican.com/article/what-is-pi-and-how-did-it-originate/)
NASA scientists were asked how many decimals of Pi they use when calculating the course of our space probes, the movement of celestial bodies and the curvature of "Space-time"!