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.

References

• http://www.x.org - X in general
• http://www.xfree86.org - XFree implementation of X (servers)
• news://comp.windows.x
• http://www.faqs.org/rfcs/fyi/fyi6.html - simple overview of papers/standards available (perhaps a bit outdated, as it was written in 1991)
• http://www.faqs.org/rfcs/rfc1013.html - X protocol
• http://www.xfree86.org/current/specindex.html - XFree collection of specification documents
• http://www.visi.com/~grante/Xtut/ - a tutorial for end-users
• http://en.wikipedia.org/wiki/X11 - the pertinent Wikipedia page

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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!)

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[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, ...]

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