Pi is almost certainly the Greek letter with the widest general recognition in the modern world, entirely because of a single mathematical constant: the ratio of a circle's circumference to its diameter, an irrational number beginning 3.14159, which the Welsh mathematician William Jones first denoted with the letter pi in 1706, a choice popularised and cemented by Leonhard Euler's later use of the same symbol through the 18th century. The choice of pi is thought to echo the Greek word perimetros, perimeter, rather than any of the letter's ancient meanings covered elsewhere in this atlas.
The constant's cultural reach now extends well beyond mathematics: Pi Day, an informal international observance held on March 14, chosen because that date is written 3/14 in the month-day format common in the United States, has been marked since 1988 and was formally recognised by the United States House of Representatives in 2009. In computer science, pi notation, written with a capital Pi, denotes a product of a sequence of terms, paralleling the summation notation built on sigma, discussed elsewhere in this atlas, and physicists use pi extensively throughout equations describing waves, circles and periodic motion.