noun

1. The sixth letter of the modern Greek alphabet (Ζ, ζ) preceded by epsilon (Ε, ε) and followed by eta, (Η, η); or the seventh letter in the ancient Greek alphabet, in which it is preceded by digamma (Ϝ, ϝ)

2. A mathematical function formally known as the Riemann zeta function.

   “Zeta of 3 is irrational”