The number π is a mathematical constant, the ratio of a circle 's circumference to its diameter, commonly approximated as 3.14159. It has been represented by the Greek letter  π  since the mid-18th century, though it is also sometimes spelled out as  pi  (/paɪ/). Being an irrational number, π cannot be expressed exactly as a fraction (equivalently, its decimal representation never ends and never settles into a permanent repeating pattern).