Most people recognize Pi from geometry class as a strange number that starts with 3.14 and goes on forever. But this mathematical constant reaches far beyond the classroom into real life, playing a vital role in everything from the smartphone in your pocket to the satellites orbiting above. Pi is an irrational number. That means it goes on forever and it never repeats its sequence of numbers. Pi has been calculated to more than one trillion digits! But NASA scientists and engineers use far fewer digits in their calculations. Usually, the approximation of 3.14 is precise enough. Pi, in mathematics, is the ratio of the circumference of a circle to its diameter. Because pi is irrational (not equal to the ratio of any two whole numbers), its digits do not repeat, and an approximation such as 3.14 or 22/7 is often used for everyday calculations. The number π (/ paɪ / ⓘ; spelled out as pi) is a mathematical constant, approximately equal to 3.14159, that is the ratio of a circle 's circumference to its diameter. 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. Then the circumference (all the way around the circle) is 3.14159265. a number known as Pi. Pi (pronounced like pie ) is often written using the Greek symbol π. With a diameter of D the circumference is πD: The definition of π is: of a Circle. The constant pi, denoted pi, is a real number defined as the ratio of a circle's circumference C to its diameter d=2r, pi = C/d (1) = C/ (2r) (2) pi has decimal expansion given by pi=3.141592653589793238462643383279502884197. π (pi)Pi is used to represent the ratio of a circumference of a circle to its diameter, denoted with the Greek symbol π is the ratio of the circumference of a circle to the length of its. Pi (often represented by the lower-case Greek letter π), one of the most well-known mathematical constants, is the ratio of a circle’s circumference to its diameter. π π is the ratio between a circle's circumference and diameter. That is, circumference diameter = π diametercircumference = π. π π is a fundamental constant in mathematics, especially in geometry, trigonometry, and calculus.