Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 noun The sixth letter of the Greek alphabet.
from The Century Dictionary.
 noun A little closet or chamber: applied by some writers to the room over the porch of a Christian church where the porter or sexton lived and kept the church documents.
 noun The sixth letter of the Greek alphabet, corresponding to the English Z.
 noun Zu + Zv − Z (u +
v ) = ksn u. sn verb sn (u +v ).
from the GNU version of the Collaborative International Dictionary of English.
 noun A Greek letter [ζ] corresponding to our z.
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 noun The name of the sixth letter of the modern
Greek alphabet (Ζ ,ζ ) preceded byepsilon (Ε ,ε ) and followed byeta , (Η ,η ); or the seventh letter in the ancient Greek alphabet, in which it is preceded bydigamma (Ϝ ,ϝ )  noun mathematics A mathematical function formally known as the
Riemann zeta function .
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 noun the 6th letter of the Greek alphabet
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
from Wiktionary, Creative Commons Attribution/ShareAlike License
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Examples

Riemann had found one very special imaginary landscape, generated by something called the zeta function, which he discovered held the secret to prime numbers.
The Elegant Variation: TEV 2007

Riemann had found one very special imaginary landscape, generated by something called the zeta function, which he discovered held the secret to prime numbers.

For the prefix challenged, a zeta is a thousand exas.

He realized that he could use something called the zeta function to build a landscape where the peaks and troughs in a threedimensional graph correspond to the outputs of the function.
SEEDMAGAZINE.COM 2009

In analytic number theory, a multiplicative version of the FourierLaplace transform is often used, namely the zeta function, can be expressed neatly in terms of Mellin transforms. of complex numbers to a formal Laurent series then this becomes a (formal) Fourier series expansion on the unit circle.
What's new 2009

Well, that may be true of some of the more general zeta functions (like Ihara), but this article is about Riemann's zeta, which is.
Citizendium, the Citizens' Compendium  Recent changes [en] 2008

I then looked up and said, "I guess I'll call you 'zeta' now."

Better would be to link them to infinite series, in case, say, they just heard about some famous problem called the "Riemann Hypothesis" involving something called "zeta", but they haven't encountered infinite series before they reach the page.
Citizendium, the Citizens' Compendium  Recent changes [en] 2008

Better would be to link them to infinite series, in case, say, they just heard about some famous problem called the "Riemann Hypothesis" involving something called "zeta", but they haven't encountered infinite series before they reach the page.
Citizendium, the Citizens' Compendium  Recent changes [en] 2008

Rcd3 (CG8231) is homologous to the human Tcomplex protein 1 subunit zeta, which is needed for proper tubulin folding [
madmouth commented on the word zeta
also an old nation in (roughly) presentday Montenegro.
May 4, 2009
ruzuzu commented on the word zeta
"3. Zu + Zv − Z (u + v) = ksn u. sn verb sn (u + v)."
Cent. Dict.
September 6, 2011