Pi is a mathematical constant representing the ratio of a circle's circumference to its diameter, approximately equal to 3.14. Calculating Pi can be done using various methods, including geometric approaches, infinite series, and algorithms. One popular method for calculating Pi is the use of the Leibniz formula, which expresses Pi as an infinite series: 4/1 - 4/3 + 4/5 - 4/7 + ... This series converges to Pi as more terms are added, allowing for increasingly accurate approximations of this important mathematical constant.