ζῆτα

indecl., ζ, τό
A. the letter ζ, Pl. Cra. 419b, etc.; of the sixth book of the Iliad, POxy. 930.15 (ii/iii A.D.), Epigr.Gr. 1095. (Semitic zayin, influenced by the names of the following letters, Gr.ἦτα, θῆτα.)
Liddell, Scott & Jones
A Greek-English Lexicon, 1940
An open-access project