X usually refers to the X Window System, currently at Version 11 Revision 6. Hence often also refered to as X11, or X11R6.
This X11 is not to be confused with MacOS X which is also often just called "X" by Mac afficiandos.
Xlib is the interface to the X11 protocol. Tk emulates many Xlib functions on platforms that don't have X11. In this way Tk has a virtual machine.
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