A capital Greek letter Zeta commonly used for special functions, particularly the Riemann zeta function in number theory.
Examples
Riemann zeta function notation
\Zeta(s) = \sum_{n=1}^{\infty} \frac{1}{n^s}
Special values of the zeta function
\Zeta(2) = \frac{\pi^2}{6}
Zeros of the zeta function
\Zeta(\frac{1}{2} + it) = 0 \text{ for certain } t \in \mathbb{R}