The Collaborative International Dictionary of English v.0.48:

Zed \Zed\, n. [F., probably through It. zeta, fr. L. zeta. See
   Zeta.]
   The letter Z; -- called also zee, and formerly izzard.
   "Zed, thou unnecessary letter!" --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

zed
    n 1: the 26th letter of the Roman alphabet; "the British call Z
         zed and the Scots call it ezed but Americans call it zee";
         "he doesn't know A from izzard" [syn: Z, z, zee,
         zed, ezed, izzard]

The Free On-line Dictionary of Computing (30 December 2018):

Zed

   1978.  Software Portability Group, U Waterloo.  Eh, with
   types added.  Similar to C.  Implementation language for the
   Thoth realtime operating system.  Added a few simple types
   for greater efficiency on byte-addressed machines.  String
   constants in case statements.  Enforces the naming convention:
   MANIFESTS, Externals and locals.  "Porting the Zed Compiler",
   G.B. Bonkowski et al, SIGPLAN Notices 14(8):92-97 (Aug 1979).