window-system

window-system is a variable defined in `C source code'.
Its value is

nil

It is a terminal-local variable; global value is the same.

Documentation:
Name of window system through which the selected frame is displayed.
The value is a symbol:
nil for a termcap frame (a character-only terminal),
'x' for an Emacs frame that is really an X window,
'w32' for an Emacs frame that is a window on MS-Windows display,
'ns' for an Emacs frame on a GNUstep or Macintosh Cocoa display,
'pc' for a direct-write MS-DOS frame.

Use of this variable as a boolean is deprecated. Instead,
use `display-graphic-p' or any of the other `display-*-p'
predicates which report frame's specific UI-related capabilities.