Info node "(lispref)System Environment" says:
-----8<-----8<-----
 -- Function: system-name
     This function returns the name of the machine you are running on.
          (system-name)
               => "prep.ai.mit.edu"

   The symbol `system-name' is a variable as well as a function.  In
fact, the function returns whatever value the variable `system-name'
currently holds.  Thus, you can set the variable `system-name' in case
Emacs is confused about the name of your system.  The variable is also
useful for constructing frame titles (*note Frame Titles::).
----->8----->8-----
But both 21.4 (.21) and 21.5 (.33) say that there is no such variable as
system-name.  (Though the function does work as advertised.)

(Thanks to Alan from fnal.gov for raising the issue on comp.emacs.xemacs
a couple years ago.)