The currently known value of the fine-structure constant (or more precisely, its reciprocal), often denoted by the Greek lowercase letter alpha (α). The constant is dimensionless (and so is the reciprocal), which means that the constant does not rely on any man-made units.
The followers of the fine-structure constant has formed a cult, and they found many formulas to approximate the constant. The fact that the constant is unitless and its connection with the fundamental laws of subatomic physics has made people think that this constant is special and maybe even divinely determined.
For a detailed coverage about the fine-structure constant and its cult, see this article by Robert Munafo.