William Jones is credited with introducing the mathematical symbol π for the circle constant.
Jones, a Welsh mathematician, used the Greek letter π in his 1706 book Synopsis Palmariorum Matheseos. He selected the symbol because it is the first letter of the Greek word perimetros, meaning perimeter, and because the constant relates a circle’s circumference to its diameter.
The symbol was later popularised by Leonhard Euler, whose influential mathematical work helped make π standard notation. This creates a common mix-up: Euler did not originate the symbol, although he played a major role in spreading its use.
The value of π is approximately 3.14159 and is irrational, meaning its decimal expansion never ends or repeats in a fixed pattern. It also appears throughout geometry, trigonometry, physics, and engineering, far beyond calculations involving ordinary circles.