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

ZetaLisp

    The Maclisp dialect used on the LISP Machine.
   The many extensions to Maclisp include vectors, closures,
   flavors, stack groups, locatives, and invisible
   pointers.  Currently supported by Lisp Machines, Inc. and
   Symbolics.

   ["LISP Machine Manual", D. Weinreb and D. Moon, MIT AI Lab,
   1981].

   (1997-03-18)