The symbol p (pi — pronounced pie) is a Greek letter that stands for the ratio of the circumference of a circle to its diameter. Here’s the approximate value of p:

p ≈ 3.1415926535 ...

Although p is just a number — or, in algebraic terms, a constant — it’s important for several reasons:

• Geometry just wouldn’t be the same without it. Circles are one of the most basic shapes in geometry, and you need p to measure the area and the circumference of a circle. So if aliens land in your cornfield and you want to measure the resulting crop circles, or if you just want to know the area of your round kitchen table, p can come in handy.

• Pi is an irrational number, which means that no fraction that equals it exactly exists. Beyond this, p is a transcendental number, which means that it’s never the value of x in a polynomial equation (the most basic type of algebraic equation). Thus, even though p emerges from a very simple operation (measuring a circle), it contains a deep complexity that numbers such as 0, 1, –1, 1⁄2, and even the square root of 2 don’t share.

• Pi is everywhere in math. It shows up constantly (no pun intended) where you’d least expect it. One example is trigonometry, the study of triangles. Triangles obviously aren’t circles, but trigonometry uses circles to measure the size of angles, and you can’t swing a compass without hitting pi.