Pi is a number.
A mathematical constant is a special number , usually a real number , that is `` significantly interesting in some way '' . Number theory or , in older usage , arithmetic is a branch of pure mathematics devoted primarily to the study of the integers . The number is a mathematical constant , the ratio of a circle 's circumference to its diameter , commonly approximated as 3.14159 . It was calculated to seven digits , using geometrical techniques , in Chinese mathematics , and to about five digits in Indian mathematics in the 5th century AD .