It's a Greek letter. Greek letters are often used to express constants (mathematical values that don't change).
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The symbol 'π' is the Greek letter 'p' which is called "pi", like our 'p' is called 'pee'.
Ferdinand von Lindemann, in 1882 happy pi day!
Pi is (probably) an abbreviation for a Greek word meaning "perimeter" or "periphery" ... the original source is not exactly clear.
Pi
"Pi" is the symbol almost always used to represent the ratioof the circumference of any circle to its diameter.