pi (π)
/ PIE /
Pi is the number you get when you walk all the way around a circle and compare that distance to the width straight across it. Roll any circular thing — a coin, a dinner plate, the Moon — and the distance around its edge is always about 3.14159 times the distance across. Not 3, not 4, but that same stubborn little number, every single time, no matter how big or small the circle.
That constancy is the whole point: pi is baked into the very shape of "round." It's why we can write it once and reuse it forever — a circle's circumference is π times its diameter, and its area is π times the radius squared. Builders, astronomers, and engineers lean on this so they don't have to re-measure every wheel and dome from scratch.
Here's the strange part: pi is irrational, meaning its decimals run on forever without ever falling into a repeating pattern. 3.14 is just a rounded snapshot, not the real value — the true number can never be written down in full. And pi refuses to stay in its lane: it surfaces in waves, in bell-curve statistics, and deep in physics, far from any visible circle.
A circle's circumference is pi times its diameter; its area is pi times the radius squared.
The symbol π — the first letter of the Greek word periphereia (περιφέρεια), "the way around" or "periphery" — was first used for this ratio by William Jones in 1706 and popularized by Leonhard Euler from the 1730s on. But pi itself had been chased for millennia: around 250 BCE Archimedes pinned it between 3¹⁰⁄₇₁ and 3¹⁄₇ by squeezing a circle between two many-sided polygons.