
Leonhard Euler, a brilliant mathematician and number theorist, not only formulated key relationships of π, but popularized the use of the symbol itself.
Giving Pi not just a value, but a name: Seventeeth century English mathematicians William Oughtred, Isaac Barrow, and David Gregory had all used the Greek letter π as a symbol for the the circumference, or periphery, of a circle. In 1706 William Jones, a Welsh mathematician, gave π its first modern usage  symbolizing the ratio of a circle’s circumference to its diameter. But it was when Euler, with his considerable prestige, adopted this symbol, circa 17367, that π came into common usage, as we know it.
