The symbol for pi is π. But why?
In 1706, William Jones, an English mathematician, first used the symbol when he stated: 3.14159 = π. But why?
Here is the standard story: A circle with diameter 1 has circumference 3.14159.... The Greeks used the word "periphery" rather than "circumference." And, the Greek word peripheria or περιφέρεια, begins with the letter π.
To remphasize the connection, if the diameter of a circle is 1, its "periphery" is π. This idea was adopted by Leonhard Euler in a 1737 publication, and the rest is history.
As a side tidbit, the value of π = 3 appears in the Bible...see I Kings, Chap. 7, verse 23). This value is maintained via a connection with the Anglo-Saxon and Old English words for "yard."
That is, a "yard" encloses a space "girt" by a fence...and if the enclosed space was a circle of diameter 1 foot, the "girth" = 3 feet or 1 yard.
Source: J. Thompson. Geometry for the Practical Man. 1934, pp. 281-82