The mathematical value of π, or *pi*, begins "3.14..." so March 14 (3/14) is considered to be International Pi Day.

*Pi* is the English spelling of the sixteenth letter of the Greek alphabet. It means "the symbol π denoting the ratio of the circumference of a circle to its diameter," and also means "the ratio itself : a transcendental number having a value rounded to eight decimal places of 3.14159265." This means that the outside measurement of a circle is a little more than three times its width. This ratio is one of the most important constants in mathematics.

*Pi* is probably used to stand for this number because it is the first letter of the Greek word that is the origin of *periphery*.

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

Merriam-Webster unabridged