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.
But why was pi chosen as the symbol for this ratio in the first place? Well, it was introduced by 18th-century Welsh mathematician William Jones, most likely, it's believed, because pi is the first letter of the Greek word for periphery (periphérion). Here's what it looks like in Greek: περιϕέρεια.
Nothing like a holiday that celebrates circumference!