pi—the ratio between the circumference and the diameter of a circle.
Calculation of pi: Archimedes around 250 B.C.E. gave the first recorded algorithm for rigorously calculating the value of the ratio between the circumference and the diameter of a circle, but he didn’t use the letter pi. Archimedes’ method was tocompute successive upper and lower bounds of the number by drawing successivelycomplex polygons inside and outside a circle.
Computation of pi took a step forward when methods for using infinite series to compute it. Here for instance is Gottfried Leibniz’ simple infinite series that converges closer and closer to π.
Use of “π” as the symbol for the ratio: The earliest known use of the Greek letter π to represent the ratio of a circle’s circumference to its diameter was by mathematician William Jones in 1706, but didn’t come into wide use until Euler started using it 1736.
Origin of the Greek Letter pi: The Greek letter pi (π) was adopted from the letter peh of the Phoenician alphabet. The earliest examples of the Greek alphabet surprisingly come from Italy. This came about because of a close trading connection between the Greek island of Euboea and the Phoencians from what is today Lebanon. The two founded trading centers in the Mediterranean including the western coast of Italy. It is here that we have the first evidence of the adoption of the Phoenician alphabet for use in writing the Greek language. The first examples of Greek letters are fragments of Homer’s Iliad and Odessy as if the phonetic alphabet was being used to record and memorize the stories that had previously been orally transmitted by travelling singers.