Pi is a mathematical constant that denotes the ratio of a circle's circumference to its diameter. It is also referred to as Archimedes' Constant, which is different from Archimedes' Number, and another name it goes by is Ludolph's Number. The mathematical value of the constant has been defined to be approximately 3.14159. . The constant derives its name from the Greek alphabet п, and is pronounced 'Pie' in English, though the equivalent written form is Pi. The Greek term is used because it is the first alphabet in the Greek words 'περιφέρεια' and 'περίμετρος', which mean periphery and perimeter respectively. It finds usage in Euclidean planar geometry, and has many uses in modern day mathematics, physics, and other such sciences.
While the approximated value of the constant Pi is now determined to be 3.14159, it is not the exact value. The exact value can be truncated to an amazing 50 decimal places and represented as 3.14159 26535 89793 23846 26433 83279 50288 41971 69399 37510. It can actually be calculated to a billions of decimal places, a practical usage is to limit it to a maximum of 10 decimal places. The decimal expansion does not end, and it does not repeat either. This is why it is called an irrational real number.