zeta

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.
3. A member of Los Zetas, a Mexican criminal syndicate.