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". It is also called Archimedes' ...

Archimedes is the attested author of the first rigorous definition of pi, which he did by way of geometry, using polygons with ever-greater numbers of sides. By 150 CE, Ptolemy had the value of pi ...