Emacs works with several window systems, most notably the X Window System. Both Emacs and X use the term “window”, but use it differently. An Emacs frame is a single window as far as X is concerned; the individual Emacs windows are not known to X at all.
This terminal-local variable tells Lisp programs what window system Emacs is using for displaying the frame. The possible values are
Emacs is displaying the frame using X.
Emacs is displaying the frame using native MS-Windows GUI.
Emacs is displaying the frame using the Nextstep interface (used on GNUstep and macOS).
Emacs is displaying the frame using MS-DOS direct screen writes.
Emacs is displaying the frame on a character-based terminal.
This variable holds the value of
window-system used for the
first frame created by Emacs during startup. (When Emacs is invoked
as a daemon, it does not create any initial
nil, except on
MS-Windows, where it is still
w32. See daemon in The GNU Emacs Manual.)
This function returns a symbol whose name tells what window system is
used for displaying frame (which defaults to the currently
selected frame). The list of possible symbols it returns is the same
one documented for the variable
Do not use
initial-window-system as predicates or boolean flag variables,
if you want to write code that works differently on text terminals and
graphic displays. That is because
window-system is not a good
indicator of Emacs capabilities on a given display type. Instead, use
display-graphic-p or any of the other
predicates described in Display Feature Testing.