Z (capital) is used in aviation, navigation and meteorology as well as by radio amateurs as an equivalent of UTC and stands for "zero longitude," that being the longitude of the Greenwich meridian. Pronounced Zulu since it's normally used within a context that requires use of the phonetic alphabet. Time thus formulated is called Zulu time. Z, rather unintuitively, has zone A one hour behind it and zone N one hour ahead.
Zeta or Zita (Ζ,ζ) is the sixth letter of the modern Greek alphabet. In older alphabets it was seventh and moved up to sixth after the digamma was dropped. It also serves as the numeral 7 in contexts in which Arabic numerals are not typically used in Greek.