Where does the symbol π come from?
In 1652, William Oughtred used π to refer to the periphery of a circle (in his expression, the ratio of circumference-to-diameter of a circle is π ⁄ δ, the latter referring to diameter).
In 1665, Jonh Wallis used a Hebrew letter 
(mem) to equal the ratio of one-quarter of circumference to diameter of a circle. (This letter plays the role as of “M” in Latin alphabets, but look how close its shape resembles a quarter of a circle, as well as the Greek letter pi !)
In 1705, William Johns used π to represent the ratio of circumference-to-diameter of a circle (believed to be first use with exactly same meaning as in today) .
From 1736, Leonard Euler, both famous and a prolific writer in mathematics works, spread the use of π in his publications.
Counting from the first relevant use, the symbol π has already had a history of more than 360 years!