The number π (/ paɪ /) is a mathematical constant. Originally defined as the ratio of a circle 's circumference to its diameter, it now has various equivalent definitions and appears in many formulas in all areas of mathematics and physics. It is approximately equal to 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 ''.
when was pi first used in its present form
since the mid-18th century