z or Z 
/ˈzi:, Brit ˈzɛd/ noun
plural z's or zs or Z's or Zs /ˈzi:, Brit ˈzɛd/ noun /ˈzi:z, Brit ˈzɛdz/ 1 : the 26th letter of the English alphabet [count] ▪ a word that begins with a z [noncount] ▪ a word that begins with z 2 Z's also Zs or z's [plural] US, informal : sleep
▪ I managed to catch/get some Z's [=get some sleep] on the flight. 3 [noncount] — used in the phrase x, y, and z to refer to the third thing in a group of three unnamed things ▪ She says that I need to do x, y, and z before I can be promoted.
▪ I managed to catch/get some Z's [=get some sleep] on the flight.
from A to Z — see 1a
