Pi is a number.
Decimal representation . A decimal representation of a non-negative real number r is an expression in the form of a series , traditionally written as a sum A mathematical constant is a special number , usually a real number , that is `` significantly interesting in some way '' . The number is a mathematical constant , the ratio of a circle 's circumference to its diameter , commonly approximated as 3.14159 . Being an irrational number , can not be expressed exactly as a fraction ( equivalently , its decimal representation never ends and never settles into a permanent repeating pattern ) . In particular , the digit sequence of is conjectured to satisfy a specific kind of statistical randomness , but to date no proof of this has been discovered .