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 afficionados.
Xlib is a higher level interface used by most applications to communicate using the X protocol. Tk emulates many Xlib functions on platforms that don't have X11.
The symbol x is often used by mathematicians, scientists and engineers to refer to the component of a vector that is parallel to the first axis of the coordinate system (the horizontal one - RS remembered the difference in school by considering that Y has a vertical bar, while X hasn't ;).
It is also sometimes taken to just refer to something unknown, as in "With added Factor X for extra cleaning power!!!" (Like much of advertising, this is of course nonsense; factors are not things you add, but things by which you multiply!)
[It probably would be useful to tell all kinds of things about X (the window system), 'bout how it's much despised in some circles, how XFree86 is only one nexus for innumerable coups and counter-coups, ...]