The 16th letter of the Greek alphabet, whose lower-case form, π, is used in math as the symbol for the ratio of the circumference of a circle to its diameter. This ratio is an irrational number, and also a transcendental number, equal approximately to 3.14159 in decimal form. A convenient fractional approximation is 22/7.
Pi plays an important role in the calculations used in LucyTuning. [Joe Monzo]