Pi is a number.
The number is a mathematical constant , the ratio of a circle 's circumference to its diameter , commonly approximated as 3.14159 .	mathematical constant	mathematical constant	ratio	ratio	circle	circle	circumference	circumference	diameter	diameter
It has been represented by the Greek letter '' '' since the mid-18th century , though it is also sometimes spelled out as `` pi '' -LRB- -LSB- pa -RSB- -RRB- .
