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/).he symbol used by mathematicians to represent the ratio of a circle's circumference to its diameter is the lowercase Greek letter π, sometimes spelled out as pi. In English, π is pronounced as pie (/paɪ/ , paɪ).