"The first mathematician to use the Greek letter π to represent the ratio of a circle's circumference to its diameter was William Jones, who used it in his work Synopsis Palmariorum Matheseos; or, a New Introduction to the Mathematics, of 1706. Jones' first use of the Greek letter was in the phrase "1/2 Periphery (π)" in the discussion of a circle with radius one. He may have chosen π because it was the first letter in the Greek spelling of the word periphery."
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"The first mathematician to use the Greek letter π to represent the ratio of a circle's circumference to its diameter was William Jones, who used it in his work Synopsis Palmariorum Matheseos; or, a New Introduction to the Mathematics, of 1706. Jones' first use of the Greek letter was in the phrase "1/2 Periphery (π)" in the discussion of a circle with radius one. He may have chosen π because it was the first letter in the Greek spelling of the word periphery."
Shamelessly plagiarized from Wilkipedia: http://en.wikipedia.org/wiki/Pi
Incidentally, π is not equal to 22/7, but that is a good approximation of the correct value, being only about 0.04% too large.
Pi is short for the greek word for perimeter. Euler used it to represent the value we know for it.
Because pies are generally round, and so are circles.
When you use Ï in geometry it is generally used to solve a problem involving circles.
So since pi is so close to pie, it only make sense that they use it.
Magic.