Today is Pi Day, an annual celebration of the famous mathematical concept that has fascinated people for millenia. But what exactly is π (pronounced like the word “pie”), and where did the concept originate?

Ostensibly, π is the 16^{th} letter of the Greek alphabet, but in mathematics it is used to represent a special number—the ratio of a circle’s circumference (the distance around the edge of circle) to its diameter (the distance from one edge to another measuring straight through the center).

Pi—which can be found in nearly all areas of mathematics and physics—is a so-called mathematical constant, meaning however big or small the circle is, its value will always be the same—roughly 3.14159.

But pi is also what’s called an irrational number—a “non-repeating” number with infinite value, or in other words, the numbers after the decimal place go on forever. Non-repeating in this sense means that no single sequence of digits—let’s take ‘546’ as an example—will ever be repeated forever. The sequence 546 may be repeated many times in a row—this is likely at some point given that pi is infinite—but at some point this repetition will stop.