**Pi is a mathematical constant and irrational number representing the ratio of a circle's circumference to its diameter with a value of approximately 3.1416.** Even though mathematicians using computers are able to calculate an approximate value for pi to 2.6 trillion digits, its exact value remains unknown. The symbol for this constant is the lowercase Greek letter pi.

It is possible to calculate the area of a circle by multiplying the square of its radius by pi. In geometry, pi is used to calculate surface area and volumes of cylinders, cones and spheres.

Trigonometry defines pi as the smallest value for which sine(x) is equal to zero, which integrates the number into wave functions for use in the natural sciences. Physicists include the value of pi in some units, such as Planck units, to eliminate their need for using it repeatedly in calculations. In statistics, distribution formulas, including the normal distribution curve and standard deviations use the constant pi.

Pi is the first letter in the Greek word perimeter. While the pronunciation "pie" is common for the United States, the Greek pronounce this letter of the alphabet as "pi." The Unicode character U+03C0 produces the lower case Greek letter for pi.