noun

• The sixth letter of the modern Greek alphabet (Ζ (Z), ζ (z)) preceded by epsilon (Ε (E), ε (e)) and followed by eta, (Η (I), η (i)); or the seventh letter in the ancient Greek alphabet, in which it is preceded by digamma (Ϝ (W), ϝ (w))
• (mathematics) A mathematical function formally known as the Riemann zeta function.
• (historical) medieval state roughly equivalent to modern Montenegro.
• A member of Los Zetas, a Mexican criminal syndicate.
• (physics) Acronym of zero-energy thermonuclear assembly/apparatus (a torus-shaped reactor used in the mid-20th century to carry out experiments in plasma physics and temperature-controlled thermonuclear reactions)