Constants

About

"A mathematical constant is a special number that is "significantly interesting in some way". Constants arise in many areas of mathematics, with constants such as e and π occurring in such diverse contexts as geometry, number theory, and calculus.

What it means for a constant to arise "naturally", and what makes a constant "interesting", is ultimately a matter of taste, and some mathematical constants are notable more for historical reasons than for their intrinsic mathematical interest."

-- https://en.wikipedia.org/w/index.php?title=Mathematical_constant&oldid=858159885

For more fun, see fbharjo's list of constants (which I find myself constantly revisiting).