pi

/paɪ/

/paɪ/

In math, pi is the ratio of a circle's circumference to its diameter. No matter how large or small a circle is, this ratio will always equal pi, or approximately 3.14.

Although it sounds just like pie, this kind of pi isn't a delicious dessert with a pastry crust. Instead, it's an irrational number, one that can't be written as a simple fraction but instead goes on infinitely after the decimal point. Rounded to 3.14, it's close enough to help you calculate the diameter, radius, or area of a circle in your geometry class. Pi is an abbreviation of the Greek periphereia, "periphery."

Definitions of pi

1. noun

   the ratio of the circumference to the diameter of a circle; approximately equal to 3.14159265358979323846...
   type of:

   transcendental number

   an irrational number that is not algebraic
2. noun

   the 16th letter of the Greek alphabet
   type of:

   alphabetic character, letter, letter of the alphabet

   the conventional characters of the alphabet used to represent speech