A Greek letter commonly used to represent the Riemann zeta function and related concepts in analytic number theory.
Examples
Riemann zeta function
\zeta(s) = \sum_{n=1}^{\infty} \frac{1}{n^s}
Zeros of the zeta function
\zeta(2n) = (-1)^{n+1} \frac{B_{2n}(2\pi)^{2n}}{2(2n)!}
Euler product formula
\zeta(s) = \prod_{p \text{ prime}} \frac{1}{1-p^{-s}}