X /X/ n.
XEROX PARC /zee'roks park'/ The famed Palo Alto Research Center. For more than a decade, from the early 1970s into the mid-1980s, PARC yielded an astonishing volume of groundbreaking hardware and software innovations. The modern mice, windows, and icons style of software interface was invented there. So was the laser printer and the local-area network; and PARC's series of D machines anticipated the powerful personal computers of the 1980s by a decade. Sadly, the prophets at PARC were without honor in their own company, so much so that it became a standard joke to describe PARC as a place that specialized in developing brilliant ideas for everyone else.
The stunning shortsightedness and obtusity of XEROX's top-level suits has been well anatomized in "Fumbling The Future: How XEROX Invented, Then Ignored, the First Personal Computer" by Douglas K. Smith and Robert C. Alexander (William Morrow & Co., 1988, ISBN 0-688-09511-9).
XOFF /X-of/ n. Syn. Control-S.
XON /X-on/ n. Syn. Control-Q.
xor /X'or/, /kzor/ conj. Exclusive or. `A xor B' means `A or B, but not both'. "I want to get cherry pie xor a banana split." This derives from the technical use of the term as a function on truth-values that is true if exactly one of its two arguments is true.
xref /X'ref/ vt., n. Hackish standard abbreviation for `cross-reference'.
XXX /X-X-X/ n. A marker that attention is needed. Commonly used in program comments to indicate areas that are kluged up or need to be. Some hackers liken `XXX' to the notional heavy-porn movie rating. Compare FIXME.
xyzzy /X-Y-Z-Z-Y/, /X-Y-ziz'ee/, /ziz'ee/, or /ik-ziz'ee/ [from the ADVENT game] adj. The canonical `magic word'. This comes from ADVENT, in which the idea is to explore an underground cave with many rooms and to collect the treasures you find there. If you type `xyzzy' at the appropriate time, you can move instantly between two otherwise distant points. If, therefore, you encounter some bit of magic, you might remark on this quite succinctly by saying simply "Xyzzy!" "Ordinarily you can't look at someone else's screen if he has protected it, but if you type quadruple-bucky-clear the system will let you do it anyway." "Xyzzy!" Xyzzy has actually been implemented as an undocumented no-op command on several OSes; in Data General's AOS/VS, for example, it would typically respond "Nothing happens", just as ADVENT did if the magic was invoked at the wrong spot or before a player had performed the action that enabled the word. In more recent 32-bit versions, by the way, AOS/VS responds "Twice as much happens". See also plugh.