It has been represented by the Greek letter “π” since the mid-18th century, though it is also sometimes written as “pi”. For thousands of years, mathematicians have attempted to extend their understanding of pi, sometimes by computing it's value to a high degree of accuracy.
In the 20th and 21st centuries, mathematicians and computer scientists discovered new approaches that- when combined with increasing computational power – extended the decimal representation of pi to, as of late 2011, over 10 trillion digits!
However, only a handful of digits are needed for most applications – only 11 decimal places are needed to calculate the circumference of the Earth to a millimeters, while only 42 are needed to find the circumference of a circle the size of the entire universe to within less than the diameter of a single proton!
So learn pi!