Pi: Welsh mathematician William Jones introduced the use of the Greek letter for the famous ratio.

Why Is the Greek Letter Pi the Symbol for That Mathematical Ratio Anyway?

A Blog About Language
March 14 2014 5:12 PM

Why Is the Greek Letter Pi the Symbol for That Mathematical Ratio Anyway?

If the spike in lookups of pi at Merriam-Webster.com are any indication, many caught wind that today was Pi Day but weren't quite sure why. Arguably the nerdiest cause for celebration, Pi Day falls on March 14, or 3/14, because the mathematical value of π begins "3.14."

Pi is the English spelling of the sixteenth letter of the Greek alphabet. It means both "the symbol π denoting the ratio of the circumference of a circle to its diameter" and "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, a ratio that is one of the most important constants in mathematics.