This is a zeta. It's the sixth letter of the Greek alphabet. In classical Greek it has the sound of z, dz, or zd. In some dialects, it is replaced with σδ. You can use it to represent an angle, or the Riemann zeta function, or a swirling piece of confetti.
You can write a zeta in HTML like so: ζ. A capital zeta is Ζ, but your browser may render that identically to the English capital Z. Make your own comparison: Ζ Z
The Greek letter preceding ζ is ε. The letter following ζ is η.
This has been a classical nodeshell rescue