"The number
π (
/paɪ/) is a
mathematical constant. It is defined as the
ratio of a
circle's
circumference to its
diameter, and it also has various equivalent definitions. It 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, and is spelled out as "
pi". It is also referred to as
Archimedes' constant.
[1][2][3]"
From
https://en.wikipedia.org/wiki/Pi which also has a neat graphic.