And Solaris and BSD confuse it further by not having an X in the title. As there are UNIXes that don't run X Window I'm not sure that it is true to say that the X in UNIX comes from there (although the converse may well be true).

My understanding of the origin of the word UNIX is that it is a pun on MULTICS, from which it was derived. I think that Brian Kerighan said that it was "one of whatever Multics was many of".