Zeta

Noun

 * 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)  A mathematical function formally known as the Riemann zeta function.
 * Zeta of 3 is irrational