Noun

Singular zeta

Plural zetas

zeta (plural zetas)

1. The name of the sixth letter of the modern Greek alphabet (Ζ, ζ) preceded by epsilon (Ε, ε) and followed by eta, (Η, η)
2. The name of the seventh letter in the ancient Greek alphabet, in which it is preceded by digamma (Ϝ, ϝ)
3. (mathematics) A mathematical function formally known as the Riemann zeta function.

   Zeta of 3 is irrational

Zeta (uppercase Ζ, lowercase ζ; Greek: Ζήτα [ziːta] Zita) is the sixth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 7. It was derived from the Phoenician letter Zayin . Letters that arose from Zeta include the Roman Z and Cyrillic З (Ze).

Unlike the other Greek letters, this letter did not take its name from the Phoenician letter it was derived from; it was given a new name on the pattern of beta, eta and theta.

Zeta has the numerical value 7 rather than 6 because the letter digamma (also called 'stigma' as a Greek numeral) was originally in the sixth position in the alphabet.

Zeta can be said to be the origin of the most common pronunciation of the Roman letter Z.