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CHrist. The letter ex here might be thought of as representing the cross, but historically stems from the Greek letter chi, first letter of the word christos, `anointed.'

Constantine famously adopted a chi-rho standard (first two letters).

Cross, as part of a word or part of a phrase or hyphenate. Examples:
  1. Xbar for crossbar (switch)
  2. X-country
  3. LaX for lacrosse
  4. In some (mostly British, that I'm aware of) place abbreviations: King's Cross and Charing Cross railway stations in London are always abbreviated KX and CX respectively.

In Henry Festing Jones's Samuel Butler: Author of Erewhon (1835-1902): A Memoir (London: MacMillan, 1919), Jones reported the following little epiphany (ch. 19, p. 363). He found a copy of Erewhon (2/e) in a second-hand book shop, inscribed by the author. (He got it for 4/6 -- the bookseller didn't recognize the hand.)

``The Book of Machines'' is in three chapters, and the second of these chapters contains a passage about looking down from a high place upon crowded thoroughfares and being reminded by the moving people of the blood corpuscles travelling through our veins. ``No mention shall be made of sewers ... nor of the yawning jaws of the railway stations, whereby the circulation is carried directly into the heart,--which receive the venous lines and disgorge the arterial, with an eternal pulse of people.'' In the copy I bought Butler has underlined the words ``yawning jaws'' and written in the margin: ``See Cannon Street and Charing Cross railway stations from the bridges or the Embankment.''

I do not know when he made the notes, but I at once remembered this passage in the introduction to Alps and Sanctuaries: ``When, again, I think of Waterloo Bridge, and the huge, wide-opened jaws of those two Behemoths, the Cannon Street and Charing Cross railway stations, I am not sure that the prospect here is not even finer than from Fleet Street. ... How like it all is to some great bodily mechanism of which the people are the blood.''

Both of the stations mentioned are terminals that trains reach immediately after crossing the Thames. Thus, someone close to the river -- on a bridge or the Embankment -- is looking at the outer end of the station, where the trains come and go.

If the station has an overall roof, the wide opening below it for the tracks might indeed resemble ``yawning jaws.'' All the more so if the roof has the classical form of a great arch. (Many major stations in Europe have such an arch over the platforms. In North America they are rare, although I think the Union station in Washington, DC, is an example.)

Charing Cross and Cannon Street stations were opened in 1864 and 1866 respectively, and indeed had arched roofs -- spanning 6 and 8 tracks, resp. Alps and Sanctuaries was published in 1881. Butler did not live to see the collapse of the roof at CX in 1905. It was replaced by a low roof; in 1990, this was in turn was replaced by a modern development over the station, intended to be reminiscent the original station design. The roof at Cannon Street was heavily damaged by WWII bombing and never replaced. Some years later there was a development like that at CX.


A designation of the standard time eleven hours behind universal time (UT), and of the zone for which it is the local time. It's the standard time on an island or two in the middle of the Pacific, and it's nautical time off-shore in the lune centered on the meridian 165° west of the prime meridian; that is, the area between 157.5° W and 172.5° W. The western third of Alaska (roughly speaking) lies within this lune, but Alaska Standard Time (AKST) is GMT-9 (time zone V), and Alaska Daylight Time (AKDT) is GMT-8.

Talking with my mom one day, I learned that her dentist didn't take appointments for Wednesdays. I couldn't resist exclaiming -- ¡¿Por qué miércoles no trabaja el dentista?!

Formally, this is one way to ask `Why doesn't the dentist work on Wednesday!?' In fact, that's how my mom at first understood it. However, miércoles also serves as a euphemism, and an equally valid alternative translation is `Why the hell isn't the dentist working?' The euphemism is based on the similar sounds of miércoles and mierda. The latter word means `shit' and is a common interjection. I'd say it's intermediate in impact between shit and merde.

Just to kill the frog, let me add that I could have added an article to clip the joke. With el miércoles the sentence would have to be understood with `on Wednesday,' `this Wednesday,' or `every Wednesday.' Los miércoles would force `on Wednesdays.' The euphemism doesn't usually take an article, but when it does it is la, corresponding to the gender of mierda.

Tossing the frog in a blender: Spanish nouns ending in s (like miércoles) generally have plural forms (like miércoles) identical with their singular forms. (There will be a quiz later in this glossary entry.) Hence, tesis means `thesis' or `theses' (the pattern holds for every other Greek -sis word I can think of). It's kind of like series in English. (In Spanish the cognate lemma has singular and plural forms serie and series, respectively.)

Maybe I should take the opportunity of a natural pause in the flow of this entry to mention that I originally planned to define the one-letter head term of the entry in the final paragraph. Unfortunately, I've completely forgotten what the entry was originally about. The x plurals in French? I don't know, and I don't believe what I've heard about their origin.

Another large class of Spanish nouns whose singular forms end in s is represented by parabrisas -- `windshield' or `windshields.' This pattern of word formation (verb + noun: para + brisas: `stop' + `breezes') is much more common in Spanish than it is now in English, where the most salient examples I can think of are scarecrow, breakwater, and more obscurely stopgap. There are also some pure adjectives of this form (as opposed to nouns used only attributively), like cut-throat. Many such terms have gone out of use without similar coinages replenishing their numbers. Among those at or past the point of extinction is break-bone fever (now better known as dengue [fever], from Swahili). The modern style is noun + verb + -er. A sort of paradigm is presented by the nineteenth-century term break-circuit, already obelized by a hyphen, that meant essentially an electrical switch. The term was not replaced, but perhaps displaced, by circuit breaker. Then again, the noun windbreak and the idiom break wind manage to coexist. I don't know what to say, but I won't consider nothing.

In Spanish, as a rule, the noun that is the last element of such a compound is used in its plural form. (I write ``last'' rather than ``second'' to include words like limpiaparabrisas, `windshield wiper[s].' A friend of mine in grad school was fond of this word. When I commented that pretty much every Spanish word is, somewhere in the Hispanophone world, a euphemism for some sexual obscenity, he wanted to know if this was specifically true of limpiaparabrisas. I'd still have to guess yes. Then again, maybe the last element of limpiaparabrisas is its second -- parabrisas -- necessarily in its plural form. Okay, sorry I mentioned all that.)

The other four weekdays besides miércoles also end in s: lunes and martes, and after the hump day jueves and viernes. All the names are derived from the Latin names of their, uh, celestial honorees, in the genitive forms used to modify the Latin dies (`day'). Hence Iovis (implicitly Iovis dies, `Jupiter's [aka Jove's] day' evolved through Low Latin into Modern Spanish jueves. Now in some other Romance languages, the Latin dies was retained in the day name. Thus, in Italian, the weekdays are lunedì, martedì, mercoledì, giovedì, and venerdì. Similarly, in French, lundi, mardi, mercredi, jeudi, vendredi. In Catalan, no doubt just to be different, the word order is reversed. (Not that there's anything wrong with that! In fact, dies fubaris was the common Classical Latin pattern, while fubaris dies is considered Vulgar.) Anyway, the forms are dilluns, dimarts, dimecres, dijous, divendres (for `day of the moon,' etc.), and very similar to the Occitan forms.

You may have noticed that the final s of Latin dies never seems to give rise to an s in any of the daughter languages. There's a good reason for this: the daughter languages drastically simplified the noun morphology inherited from Latin. Typically, in modern Romance only the oblique (i.e., objective) singular forms have survived to provide the roots in the new noun system. It's something like the replacement of ``He and I went'' by ``Him and me went'' and similar horrors of illiterate English, or moi in place of je as predicate [previously] nominative in French.

More particularly, the Latin form that became the root of the Romance noun was usually, depending on the language, consistently either the accusative or the ablative. The final m of the Latin accusative was already being elided in Late Roman speech, so it too disappeared, and with all the vowel shifting it is sometimes a close call whether the accusative or the ablative dominated, and historical data is usually recruited to make the call. It's pretty clearly accusative for French (coexisting with nominative in Old French) and ablative for Spanish. [Moreover, in all modern Romance languages that I've tried to investigate, the only noun inflection is a plural form that is pretty regularly formed (with the usual assorted peculiarities, such as the final -s singular-plurals discussed here).] Anyway, dies was already a pretty oddball noun in Latin, even standing out from the crowd of other fifth declension weirdos, but bottom line, its singular forms had no s except in the nominative and vocative. (Day! O day?)

So between the parallels with other languages and the more general patterns, it should be clear that the final s in Spanish lunes, martes, etc. doesn't come from dies. In the case of jueves given earlier, it evidently comes from Iovis, the genitive being preserved in this case because it was the salient or only form continuing in use. Likewise, martes (Tuesday) and viernes (Friday) have the final s from the Latin genitives Martis and Veneris. But not all Latin genitives end (or even ended) in -is. Roman Monday was dies Lunae and Wednesday dies Mercuri. So where did lunes and miércoles get a final s? Linguists state that the s was added later by analogy with the other weekdays. Likewise dilluns and dimecres in Catalan, etc.

Now where was I? Oh yeah, quiz time. How do you say Wednesday in Spanish? Yes, miércoles, ¡very good! You might have to work on the pronunciation.

A Scrabble tile worth eight points (or more, on a double- or triple- letter or word space, or if it's used in multiple words) (or negative eight, to the holder, if someone else uses up his or her tiles first). Therefore, it behooves you to study this important resource (words in the OSPD that contain the letter X).

In every Scrabble set, exactly one of the 100 tiles is an X. The other high-value letters (one tile each) are J (also 8 pts.), and Q and Z (ten points each).

EXciton. A bound electron-hole pair. Solid state analogue of electronium (electron-positron pair; not very long-lived).

EXtension. Common abbreviation for telephone numbers within a PBX.

EXperimental. As in the MX missile, which kept its name and was never deployed. Also as in the ``X planes'' a series of US-government-funded experimental vehicles (not all planes: X-7 through X-12, at least, were missiles). The most famous X plane was the X-1, the first plane to break the sound barrier. It was originally designated the XS-1, where S reportedly stood for supersonic. It seems at least possible that XS stood for transonic (so that the expansion of X changed), but I haven't found any explicit support for that view.

Generic chemical symbol for a halogen atom (F, Cl, Br, I, At). In normal usage, the term halogen is used for the (diatomic) elements and the neutral atoms; the associated anions are called halide ions, and their salts halides.

Earlier, it was understood more in terms of its etymology (`salt former') and included at least one nonelemental species: cyanogen. Nowadays, cyanogen [the dimer (CN)2] is more likely to be called a pseudohalogen, and cyanide (CN-) a pseudohalide ion. The terms pseudohalogen and pseudohalide are applied to other carbon nitrogen compounds -- cyanate ion (OCN-), isocyanate ion (NCO-), fulminate ion (ONC-), thiocyanate ion (OCN-), thiocyanogen ((SCN)2), and cyanamide ion (NCN2-).

Hydrogen cyanide, unlike hydrogen halides (I mean nonpseudohalides) is a very weak acid,. This is bad news, because it means that cyanide salts exposed to acid react to release hydrogen cyanide, a very fast-acting poison.

I haven't checked old definitions yet, but possibly at some point in the nineteenth century, a halide was just the conjugate base of any hydro acid. One source I've seen claims that the term halogen was coined by Berzelius. That's plausible, but I have some doubts to assuage before I simply assert it as fact.

In specific instances even today, I imagine that X is used to represent halogens and pseudohalogens, or some subset. The reactions and solubilities of thiocyanates resemble those of bromides and iodides, so one might use X to represent any of those three in certain discussions. (Of course, X also serves as a generic symbol for any atomic or molecular species, when no more specific symbol is available.)

Unknown, undetermined; indeterminate. Descartes established the use of certain alphabetic characters to represent various kinds of quantities, and still today x is probably the most popular character used to represent a variable, which may be regarded as an `unknown.' The usage is not restricted to mathematics, however. For many years, Stevens Institute of Technology in Hoboken, New Jersey had an unnamed building. The building had been erected but not named in the hope or expectation that a donor would be found, after the fact, to make a large contribution in return for the opportunity to name the building. Things didn't work out, and over its many years of anomia it came to be called `Building X.' (It might have been worse; see the King's entry for an earlier naming debacle at another New Jersey school. See the T. Kirk, James entry for a related situation in a big country way up north.)

Röntgen, who discovered them in 1895, gave X-rays their name. For a long time, however, they were also known as ``Röntgen rays.'' His amazing discovery led others to seek new kinds of radiation, including ``N-rays'' which turned out to be a figment of discoverer R. Blondlot's imagination. More here.

If for some reason diacritical marks aren't being used (increasingly, this is due to ignorance rather than unavailability of the appropriate characters) Röntgen is spelled Roentgen. (In German, ä, ö, and ü are essentially interchangeable with ae, oe, and ue, respectively, for general words. People have preferences about how their names are spelled, however, so as a rule one uses a single form for each person's name and does not make the substitution unless it is unavoidable. Thus, the name of the Dichter Goethe is not normally written Göthe by any literate person.)

X is also widely used as a generic symbol in formulas of various sorts. Examples include OXR (for the government organizations like ONR) and in chemical formulas where confusion with usage in the halogen sense is unlikely (or unavoidable). An example of the latter occurs in the title and paper of L. W. Schroeder, M. Mathew, and W. E. Brown, ``XO4n- Ion Hydration. The Crystal Structure of Mg3(PO4)2.22H2O,'' in The Journal of Physical Chemistry, vol. 82, no. 21 (October 19, 1978), pp. 2335-2340. There, X refers to nonmetals in the nitrogen-group nonmetals phosphorus (the focus of the work, and the X for which new experimental data were reported) and arsenic, and the chalcogenide sulfur.

X, X-band
Microwave band that is lowest mode in 1'' waveguide. Vide microwave band names.

Negative. The letter ex used in various negative senses does not abbreviate any particular word. That is, it does not function acronymically, but perhaps ideographically, representing a barrier or crossed arms, symbolizing some sort of avoidance, and implying danger, trouble, or some kind of disagreement or disagreeableness. Hence also XXX.

In his diaries, William E. Gladstone (1809-1890) marked with an ex his meetings with women who represented a moral danger -- mostly prostitutes whom he met in an, umm, missionary spirit, courting evil while doing good. He was already a prominent MP when he used the mark as a comment on his meetings in the late 1860's with Mrs. Laura Thistlewayte (a kept woman in her youth, a lay religious preacher after her marriage -- when she knew Gladstone).

Por. In Spanish, the multiplication sign × (or x if the precise symbol is not available) is read as ``por,'' presumably short for ``multiplicado por,'' just as it may be read ``by,'' short for ``multiplied by,'' in English. (The English ``times'' in this context could be rendered by ``veces,'' but I've never encountered that.) Anyway, in informal and character-count-constrained contexts like Facebook, Twitter, and text messaging, x is used as an abbreviation for the preposition por. It seems somewhat girlish to me, like dotting an i with a heart, but it does abbreviate. A local (South Bend, Indiana) Mexican restaurant has a hand-lettered sign offering ``TACOS 5x5.'' This obviously means `five tacos for five dollars' [as this would be cinco tacos por cinco dólares], but I don't know how obvious it might be to monolingual Anglophones.

In a similar way, the plus sign (+) is read as ``más'' in mathematical and scientific contexts where it would be read as ``plus'' in English. The word más has a variety of functions or senses, but its primary use is as a comparative adverb that is often translatable into English as `more.' Some of the meanings are distinguished by representation in an unaccented variant (mas); this would typically be translated as `but' or `or,' although it has only some restricted senses of these words. In parallel with the multiplication sign, the addition sign is used for the most common instances of más, but I can't recall well enough to say which of the less common senses of más I've seen it as an abbreviation for, to say nothing of mas.


Roman numeral for ten. Although Roman numerals were originally a set of symbols independent of the alphabet, the symbols evolved under the influence of the alphabet and were eventually replaced. The alphabetic letter X resembles the original symbol for ten and was also thought of as representing two letters vee (one inverted under the other) when the letter vee came to represent five.

Trans-. The Latin preposition trans has a fairly narrow range of senses that can often be translated as `across.' You can regard X as a visual pun (a cross) or as a graphical representation of the meaning.

TRANSistor. The word transistor was coined as a blend of transfer and resistor.

The first transistor was the point-contact transistor, involving two metal points contacting a semiconductor (germanium) surface. The first working transistor of this type (``type-A'') was made by Walter Brattain in December 1947. (By some definition, Walter was a Chinese physicist: he was born in Amoy on February 10, 1902, where his father had a job teaching science and math.)

Brattain recognized the importance of the transistor from the beginning. He said this in a 1964 interview:

It is of interest to those that ask whether we knew how important this was, that the evening of the first day, when John [Bardeen] had come in and suggested the geometry, I told my riding group that night, going home, that I felt that I had that day taken part in the most important experiment I had ever taken part in in my life. And the next evening going home with them I had to swear them to secrecy.

Point-contact transistors were fragile and noisy. In early 1948, William Shockley came up with the idea of the semiconductor junction transistor. After a number of crucial steps, the first efficient working transistor of this type was announced in July 1951. The following appeared in 1952, in the monthly Electronic Engineer (London), vol. 24, p. 42:

Although it is unlikely that the transistor will ultimately displace the electronic valve, there is no doubt that for many electronic applications the transistor...will be preferred because of its robust and compact form.

EXtended Architecture.

X-ray-induced Auger Electron Spectroscopy (AES).

EXchange Access Frame Relay.

X-ray Absorption Fine Structure. New name for EXAFS.

Visit the relevant scholarly society (IXS).

EXtensible Application Markup Language. Pronounced ``zammel.''

X-ray Absorption Near-Edge Structure. Not exactly another name for XAFS.

Xapia, XAPIA
X.400 Application Programming Interface (API) Association. Body standardizing the interface to X.400 email service.

Other API's, like VIM and MAPI, are expected to be Xapia-compliant once Xapia is finalized.

XARA's Are Recursive Acronyms. Some other XARA's are

See also EFS and navel exercises discussions.

Why not BARA, CARA, ....? Because I thought the exes could use some bulking up, that's why. Fascinating stuff about the XARA/sealed acronym nexus can be found at the HLN entry.

XARA's are usually intentional, and AAP pleonasms are usually accidental, but see VS Sassoon.

X-ray Absorption Spectroscopy.

EXchange Access Switched Multimegabit Data Service.

Microwave band around 3 cm wavelength. The reason for the ambiguity here is the fact that there have been a number of somewhat different designations. As I understand it, the original names come from the single-letter designations of different plumbing standards that were used for circular waveguide in the early days of research.

[Phone icon]

CrossBAR. A relay for power distribution or telephone connection on a grid or network.

EXpendable BathyThermograph.

A packet-switched bus.

X Bit Map.

CROSS-Country (skiing, for example). Also X-country.


Xerox Copy, but when the trademark police come knocking on your office door in the middle of the night, you can say it means ``xerographic copy,'' and they'll have to go away disappointed -- no arrest. But don't let down your guard! If they ask for ``scotch tape,'' you probably don't have any in the office: offer them ``transparent adhesive tape.'' ``Band-Aids, Q-tips.''? ``Plastic strips, cotton swabs.'' And that dark carbonated beverage in your paper cup? ``Cola.'' Practice safe speech.

EXtended CMOS.

X-ray Computed Tomography. This seems to be a new acronym that solves a problem that is not just old but gone. Computed tomography (CT) or computed axial tomography may in principle refer to positron emission tomography, or single-photon-emission computed tomography, or magnetic resonance imaging. In practice it mostly doesn't, since these are normally referred to as PET, SPECT (or SPET), and MRI (or NMR) respectively.


X Display (window-) Manager Control Protocol.

EXternal Data Representation (protocol).

It's Greek to me.

{EXtremely|EXtensively}-Resistant (TB). See XDR-TB, infra.

{EXtremely|Extensively} Drug-Resistant TuBerculosis. Any TB strain resistant to three or more second-line antibiotics.

This first appeared in the medical literature in 2006 as Extensively drug-resistant TB. The first article to use the term seems to have been ``Emergence of Mycobacterium tuberculosis with Extensive Resistance to Second-Line drugs -- worldwide, 2000-2004,'' in Morbidity and Mortality Weekly Report, Vol. 55, 2006, pp. 301-305. A version of the article is available online. (There are 26 individual co-authors. I'm not familiar with publication practices in biomedical research, but in my fields the last author is typically a or the group leader. Here the last author is N.S. Shah and I've seen the article referred to as Shah et al.)

The investigators found emerging resistance not only to isoniazid and rifampicin (resistance to these defines MDR tuberculosis), but also to at least three classes of second-line drugs (SLD's), which they termed extensively drug-resistant (XDR) tuberculosis.'' Almost half of TB cases treated in Russia recently have been of this type. (I have this from ``Drunken Nation: Russia's Depopulation Bomb,'' an article by Nicholas Eberstadt in the Spring 2009 World Affairs; it appears to be an estimate by the WHO for the 1999-2008 period.) Cf. MDR-TB.

X-windows-based DVI file viewer.

Chemical element abbreviation for xenon, a noble gas. Atomic number 54. Learn more at its entry in WebElements and its entry at Chemicool.

X-ray Energy Dispersive Spectroscopy. An electron beam is used to kick out electrons from atoms, and electrons in higher levels cascade down to refill the hole left behind. The X-ray photons emitted in this cascade have a spectrum of energy level separations (K, L, M, ...) characteristic of the particular atom they are emitted from; solid-state detectors can analyze this energy distribution. The X-ray energy distribution is deconvolved using the known lines and intensities of various atomic species to determine an atom composition. Accuracy is not available that can distinguish among isotopes, and atoms lighter than carbon have ``deep'' electronic energy levels too close to emit in the X-ray regime.

A commercial unix for PC. I think people tend to use Linux these days, and put X-windows over that, if they want a Unix alternative to MS Windows (Windows95, 97, ...).

A chemical substance foreign to an organism or not a part of its normal ingestion. ``Normal'' and ``foreign'' here are understood rather conservatively; I would say that the aspartame in Diet Coke is a part of my normal hourly nutrition (are you sure it's not nutritious?), and that a little bit of radioactive strontium is not foreign to my bone structure, but xenobiotic researchers beg to differ. Maybe they even demand to differ. I think they should worry about the important stuff first, like figuring out what they want to be called, as it doesn't seem to be ``xenobioticians'' or ``xenobioticists.'' I propose that they select Xena: Warrior Princess (yeah, there seems to be a colon in her name) as their goodwill ambassador, because it would be cool for her to be goodwill ambassador for anything.

But xenobiotics are not just anything either. In particular, xenobiotics are not the same as xenobiology. Too bad. I noticed that the International Society for the Study of Xenobiotics (with the sexy acronym ISSX) holds satellite meetings associated with its triennial conference. Satellite meetings! Wonderful!


After all these centuries, he's finally got a homepage. It doesn't have much content and it hasn't been updated since 1999, but that's okay. These things take time.


A trademark of Xerox Corporation of Rochester, New York. The word is widely used in the US with the meanings xerographic copy and copy xerographically. This isn't very surprising, since the company commercialized xerography. The story is told all over the net, so I only have to tell you the important stuff: Chester F. Carlson, who (with Otto Kornei) invented the process, named it electrophotography. The word xerography, from Greek roots meaning `dry' and `writing,' was proposed by a Classics professor at Ohio State University. I know some Classics professors at OSU, but I'm pretty sure it wasn't any of them.

The company that bought the rights to the process and developed it into a marketable product was Haloid, of Rochester, New York. They created the trademark Xerox in imitation of the name of its giant neighbor in Rochester, Kodak. (Kodak was one of the many companies that had turned down the chance to develop Carlson's patent.)

In 1949, Haloid introduced its first model and changed its name to Haloid Xerox. In 1959, Haloid Xerox introduced the Model 914, the first truly automatic model. They advertised it with a TV commercial in which a little girl makes a copy for her dad and then can't remember or tell which was the original and which was the copy. Many people watching the commercial couldn't believe the process was so automatic a girl could do it. Others did, and sales took off. In 1961, Haloid Xerox shortened its name to Xerox. The name Haloid is explained at the haloid entry.

Misbegotten old currency symbol (q.v.) for the old ecu.

EXit Flight Level.

EXtreme Football League. From the people who brought you the WWF, collegiate-level football plus low-rent hype. Performed on Saturdays after the NFL season, starting just as Super-Bowl hype withdrawal starts to set in. Canned after one season (2001)! Mencken may have been right that no one ever went broke underestimating American taste, but at least someone lost money.

EXtended Graphics Adapter. Better known as EGA.

Symbol used on ``Bloomberg terminals'' (a ticker service) for a ``Greek Drachma (post Euro).'' This was first noticed in late May 2012, when no such currency was current, and there was no invented trade information available for the invented currency. The Bloomberg help desk would say only that it was ``an internal function which is set up to test.'' You'll have to decide for yourself whether that's a reflexive or pointedly intransitive ``test.'' The function went away on June 1. Presumably it will come back for the widely anticipated Grexit.

EXtensible HTML. XHTML 1.0 is a reformulation of HTML 4 as an XML 1.0 application with three DTD's corresponding to the ones defined in HTML 4.

W3.org offers an XHTML validation service.

EXtreme High Vacuum. Corresponding to pressures below about 10-10 Pa (100 pPa, if you like) or 10-12 mbar.

EXchange Identification.


I've seen this abbreviation a lot, but only on the internet. Then again, maybe I just don't have a nonvirtual life, so I would have no opportunity to see it elsewhere. In that case, I must be born again!

EXchange IDentification.

EXtended Integrated DataBase.

X-Ray Imaging Experiment. Not any ol' X-ray imaging experiment, but a particular NASA project: see EOS. Not surprisingly, many years later, the initialism seems to be less common than results of X-ray imaging experiments published with anyone named Xie as a co-author.

X-ray Interference Lithography. This is explained in the entry for the more general term XRL.

According to their webpages, they're ``the world's leading supplier of CMOS programmable logic and related development system software.''


Xenon Ion Propulsion System for nudging satellites around.

Xenon (Xe) is the heaviest non-radioactive noble gas. As such, it is about the densest gas at any given pressure. [Almost anything else that heavy would have stronger intermolecular forces, and would likely not be gaseous at the low temperatures reached in the shadowed (from the sun) regions of space.]

Why am I telling you this? Consider that you're using accelerated ions, of some sort, to propel a spacecraft. Say the ions have a mass m and that they are singly ionized (this is typical) so they have a charge e (that's q to electrical engineers), and that they are accelerated through a voltage V. Thus, each accelerated ion has an energy E = qV. This is kinetic energy for the ion, with velocity u and speed u = |u|:

        1   2
       	- mu  = E .
Hence, the impulse received by the ion -- the change in its momentum, equal to the time integral of the force on it, is I = sqrt(2mE). The impulse that this exerts on the spacecraft is equal (and opposite in direction). In other words, for a given energy or voltage used, the impulse received by the satellite is proportional to the square root of the ion mass. This simply represents the well-known and intuitive fact that recoil from a heavy object is greater than from a light one.

The analysis is simplest in terms of momentum or impulse: by Newton's third law, the forces that spacecraft and ion exert on each other are equal and opposite, and -- classically -- they act over the same time, so the impulses are equal and opposite. Thus, in order to get the most momentum bang for the energy buck, you want to use the most massive ions possible.

(Relativistically, momentum is also conserved, but in a typical reference frame the force durations are not equal. This is compensated by the fact that the forces are not equal either.) The kinetic energy changes of spacecraft and ion are not equal (in the center-of-mass frame), because the work done on each object is the distance integral of the force, and the distances traveled by the objects are different. Again, however, heavier ions produce a greater recoil, so for the same force they produce a greater recoil (i.e. acceleration) of the spacecraft.

EXpansion Joint. A joint that adjusts to accommodate the (typically thermal) expansion or contraction of the pieces it joins.

X-band Klystron.

xkcd, XKCD
The name of a popular on-line comic. The creator, Randall Munroe, claims that xkcd can't be pronounced phonetically. For those who have difficulty understanding this, and for those who don't, there's an illustrated explanation.

XML Key Information Service Specification. One part (X-KRSS is the other) of XKMS.

XML Key Management Specification.

XML Key Registration Service Specification. One part (X-KISS is the other) of XKMS.


Rebus for excel. Roman numeral for 40 (but consistently only since medieval times; the Romans themselves normally wrote IIII for four, VIIII for nine, etc.). (They also doubled up sometimes, writing XIIX for 18, at least.)

EXtra Large.

X Logical Font Description.

xlock &
Screenlock in background -- performance art for the impecunious existentialist!

Cross-Linked (high-density) PolyEthylene.


XMAS, Xmas
CHristMAS. A pagan, Christian, bank, and secular holiday celebrated on Isaac Newton's birthday. It's at the center of the holiday shopping season. In the US, the official beginning of the shopping season is the day after Thanksgiving (US Thanksgiving is the last Thursday in November). In the more European-peopled parts of the Commonwealth (am I allowed to say that?), there's a Boxing Day public holiday after Christmas.

Practices vary. In Spain and Italy, children traditionally receive gifts on the Day of the Magi, though Saint Nick is evangelizing furiously there. A relevant Christmas story can be found at the Martinmas entry.

EXtensible Markup Language. Heir (nicht schon hier) apparent to HTML.

XML is closely related to HTML and SGML and assumes a very similar, but not identical syntax. One difference: in XML one cannot omit end tags. Tags for elements which don't have any content are specially marked by a slash before the closing angle bracket. Also, properly speaking, XML (like SGML) is a meta-markup language -- a set of guidelines for how markup rules should be defined. Within XML, one defines domain-specific markup languages using document type definitions (DTD's).

What, that isn't enough information to just tucker you out? Okay then, try Jon Bosak: XML, Java, and the future of the Web.

(NASA) X-ray MultiMirror (mission).

Unknown or Generic (nucleotide) MonoPhosphate. Examples: AMP, CMP.

EXtended Memory Specification.



EXclusive NOR. That is, the negation of an XOR logic function (or the inversion, as it is called in the case of a logic gate, of an XOR). Less commonly called NXOR.

Xerox Network Services. (Sometimes ``... System[s].'') Proprietary networking architecture developed by Xerox. Before your knee jerks, remember how much of what is new today was old years ago at Xerox PARC.

Nowadays, XNS has been adopted/defined/accepted as an ISO protocol for Xerox LAN's.

Xerox Network Services ID Protocol.

X-On-Insulator, where X is a compound semiconductor (typically II-VI or III-V). More precisely, a structure in which an ultra-thin film of compound semiconductor is transferred onto a Si/SiO2 handling substrate. XOI refers both to the structure and to the technology (fabrication and processing, etc.) associated with the structure. The generalized term was coined by analogy with the similar SOI (Si-On-insulator) technology.

It seems that XOI (the technology and the acronym) was first proposed in print in Ultrathin compound semiconductor on insulator layers for high-performance nanoscale transistors, H. Ko, K. Takei, R. Kapadia, S. Chuang, H. Fang, P. W. Leu, K. Ganapathi, E. Plis, H. S. Kim, S.-Y. Chen, M. Madsen, A. C. Ford, Y.-L. Chueh, S. Krishna, S. Salahuddin, and A. Javey, Nature (London) vol. 468, pp. 286-89 (11 November 2010).

An eX-Googler. I.e. a former Google employee.

EXclusive OR. The name of a logical operation and of a logic gate that performs it.

The binary XOR operation is defined to be true if precisely one of the inputs is true, and false otherwise. It is evident (by enumeration of cases) that this XOR operation is associative, and it's obvious that it's commutative. Given these two properties, it follows that one may define the XOR of a collection of operands, irrespective of order (of the operations or the operands) in the same way that one defines the sum of a collection of numbers, given the associativity and commutativity of the binary sum (addition) operation.

In general, the XOR of a set of operands is true if the number of true operands is odd, and false otherwise.

The Greek letters chi and rho. The first two letters in the word christos, `anointed.' First used as a Christian emblem by Constantine. It came to him in a dream, that under that banner he would conquer.

Xeroderma Pigmentosa. Common name for a disease involving dry skin that becomes discolored. More at the next XP entry. If you can't decide whether to sound or be right, just use the shared abbreviation.

Xeroderma Pigmentosum. See XPS. If this looks odd (i.e., if you were expecting xeroderma pigmentosa), then note that xeroderma is constructed from Greek roots, and though it ends in -a, it has neuter gender.

Xeroderma means dry skin, but this is the disease whose sufferers are mortally sensitive to sunlight. XP is a rare genetic disease (150 children in the US are diagnosed with it) that destroys DNA's ability to self-repair. Severe burns as a result of brief UV exposure are the first symptom, typically occuring in the first months. In about twenty percent of cases, there is increased risk of neurological symptoms. Read more here.

X-ray Photoelectron Diffraction.

X/Open Portability Guide.

EXplosion PRooF (motor). Totally enclosed, needless to say.

Xeroderma Pigmentosum Society.

X-ray Photoelectron Spectroscopy. An analytical method in which a surface is irradiated with monochromated X-rays, inducing the emission of electrons (called photoelectrons after the method of generation). Energy analysis (i.e., spectroscopy) of the photoelectrons gives information about the atomic composition, oxidation states, and structure of the first few nanometers of the surface.

The method was developed by Kai M. Siegbahn starting in the 1950's, and he was awarded the Nobel Prize in Physics for this work in 1981. Siegbahn called the method ``Electron Spectroscopy for Chemical Analysis'' (ESCA). XPS is the more modern name, and the ESCA name is now somewhat less common than XPS, but both are used (sometimes together, as ``XPS/ESCA''). As we clean up the glossary we expect to continue giving users the run-around by forcing you to visit the ESCA entry for further information.

EXpandable polyStyrene. Also EPolyS.

EXtruded polyStyrene. Particularly in building construction.

Software for a Graphical Console and Monitor for PVM.

EXtended Q. See Q2.

Electromagnetic radiation (i.e., light) of energy on the order of at least 1000 eV, or a wavelength not larger than about 10 Å. Discovered accidentally by Roentgen.

The Department of Physics at Uppsala University in Sweden has made available an X-Ray WWW Server.

X-ray glasses
Glasses with lenses that blur optical light to an extent that makes people viewed through them look like skeletons. Go ahead, send away for them.

When Madonna was known as Madonna Ciccone, and in high school in Michigan, she wore flesh-tone panties under her (own) cheerleader's uniform.

Madonna is younger than I, yet she is already much better known. I can deny it no longer: I am a failure.

EXplosive ReCRYSTALlization.

X-Ray Rocking Curve.

X-Ray Diffraction. Here's an introduction from Virginia Tech.


X-Ray Fluorescence.

Here's some instructional material from Virginia Tech.

X-Ray Fluorescence (XRF) Analysis.

X-Ray Fluorescence (XRF) Spectroscopy.

X-Ray Lithography. This is done in two ways. In commercial applications, X-ray lithography is essentially a short-wavelength version of optical lithography. Since the wavelength scale is considerably smaller than optical wavelengths, the resolution is potentially much finer. In practice, however, the linewidths achieved are no smaller than about 0.2 µm -- considerably larger than the diffraction limit. The main reasons for this are: (a) the difficulty of fabricating effective masks with the necessary resolution; (b) the high energy of the X-ray photons, which makes it difficult for the energy deposition, and thus the resist exposure, to be well localized; and (c) the difficulty of keeping the grown, doped, etched and deposited regions as finely defined as the X-ray exposed region.

Another application of X-ray lithography occurs in the fabrication of nanostructures. Specifically, the X-ray beam is split into a pair of aligned beams, projecting a diffraction pattern on the surface of the resist. Exposure and development produce a lattice pattern over a wide area with a pitch of as little as 1000 Å (0.1 µm). This is also called X-ray Interference Lithography (XIL).

An alternative approach, not evidently very efficient, would be to use a focused the X-ray beam. The problem with that is making an X-ray lens. Recently [reported in 1996.11.7 issue of Nature], Dr. Anatoly Snigirev and his wife Dr. Irina Snigireva developed a new lens for X-rays, and demonstated a submicron focal diameter. The new lens works somewhat like a quadrupole magnet in electron beam focusing: individual lenses create a divergence in one direction and a convergence in the perpendicular direction. Successive lenses oriented in different directions produce a net focusing in both lateral directions. An individual lens consists of an aluminum block, with a plane of parallel holes drilled in from the side.

X-Ray Microscopy.

X-Ray Microscopy and Spectromicroscopy conference, to be held 19-23 August 1996 in Würzburg.

X-Ray Reflectivity.

X-Ray Reflectometry. A surface science technique. An introduction is served by the UK ESCA Users Group.

Measurement of X-ray intensity along the undiffracted (i.e. specularly reflected) direction. Angle-dependent intensity changes arise from characteristics of surface roughness and from interlayer interference, and so angle dependence is used to measure surface roughness and layer thickness.

X-Ray { Spectrometer | Spectrometry | Spectroscopy }.

X-Ray Telescope.

X-Ray Topography.

EXcESS. Another rebus in this glossary is MT.

EXternal Subroutine interface.

X-band Synthetic Aperture Radar (SAR).

EXtended Source Catalog. A list of nebulae and stuff.

EXtended System Description Table. An ACPI system description table.

{ eXtensible | XML } Stylesheet Language.

EXperimental Stealth Tactical (warplane).

Cross-sectional STM.

X-Ray Standing Wave[s]. A surface science technique. Visit this description served by Christopher Walker.

The name is a bit of a misdirection. The idea is that the incident and Bragg-reflected X-rays set up a standing-wave pattern throughout the region penetrated by the X-rays, and above it. A surface technique such as Auger (AES) or fluorescence spectroscopy (EDX) is then used to monitor the surface as the standing-wave pattern is varied by changing the angle of the sample relative to the incident X-ray beam.

Extended Technology. Used by IBM for first PC generation after the original IBM PC (the ``PC - XT''), both now long since obsolete.

X Toolkit.

Cross-talk. You couldn't figure this out?

Cross-sectional Transmission Electron Microscopy (TEM).

EXtended Three-Letter Acronym (TLA). You think I'm kidding, don't you? It normally refers to four-letter acronyms, and it avoids the ambiguity of FLA. ETLA is a more common acronym meaning the same thing, but I disapprove.

(Synchroton-radiation) X-ray Tomographic Microscopy.

EXpress { Transport | Transfer } Protocol.

A Vietnamese unit of currency widely used in hard Scrabble lands. The word is just a Vietnamese borrowing of the French word sou, as in ``not worth a sou.'' It's worth a hundredth of a dong (VND). A US dollar is worth in the neighborhood of 20 thousand dong, so you can imagine that the xu is much more valuable in Scrabble than elsewhere.

EXtreme Ultra-Violet.

X-Window[s] (screen image) Dump.

A Mexican beer -- ``Dos Equis'' (`two exes').

Used in preparing a news story, to indicate information to be filled in later. Sometimes the story goes to press before the information is filled in.

Here's a particularly bad case of unreplaced exes that was published in the Chicago Sun-Times on July 15, 1997, pg. 6 of the main news section (Tuesday, Late Sports Final Edition):

Verneal Jimerson (above) and Dennis Williams (left) were two of the Four Heights Four convicted of xxxxx in 19xx. They were freed in 1996.

[skip a paragraph]

Joseph Burroughs: convicted in 19XX for the robbery-murder of a Downstate man; cleared in 19XX.

The Boston Globe on Bastille Day 1996 had an article on the Reform party US presidential nomination that described presidential hopeful Richard Lamm as ``the Democratic governor of Colorado from 19xx to 19xx.''

The Washington Post, on April 27, 1994, gave the publication information for Dorian Leigh Parker's Doughnuts: Over 3 Dozen Crullers, Fritters and Other Treats as ``(Clarkson Potter, 19xx, $11).'' Highlights include Sour Cream Doughnuts, Snail Buns and Potato Doughnuts.

stop the press! The Times-Picayune (New Orleans) on June 11, 2000 television guide had a review of a TBS made-for-TV movie called ``On Hostile Ground.'' It's about a sinkhole that collapses the French Quarter during Carnival. The salvation of the city is some kind of miraculous polyurethane mixture, according to a heroic geologist named XXX Andrews (John Corbett). Also, the rerun movie ``Superdome'' is glossed as (ABC, 19XX).

For a variety of applications, it is useful to have a template that one fills in, replacing text like ``Headline goes here,'' ``Caption goes here,'' and ``(Source here)'' as illustrated in the screen capture at right. When the news is hot, you may see stuff like this posted on a website.

See also 000 and TK.

Hard-core pornography. See X.

Poison. See X.

An Australian beer. Nicknamed ``barbed wire.''

EXperimental geometrY Zürich. Developed at Zürich. Moreover, it is used for three-dimensional description, whose cartesian coordinates are conventionally x, y, and z. So clever!

Version 11 of the X protocol.

Version 11, release 4 of the X protocol.

Version 11, release 5 of the X protocol.

Vide ASC X12.

CCITT recommendation X.200 is the basic reference model OSI. It's always called OSI, never X.200.

International (CCITT) standard protocol for communication between a circuit-switched network and user devices.

International (CCITT) standard protocol for serial data transfer in packets. A standard for OSI model layers 1-3.

International (CCITT) standard for terminal-PAD interface.

International (CCITT) standard for PAD-computer interface.

The C++ standard technical committee.

The international standard protocol for electronic mail. Strictly speaking, X.400 is the CCITT recommendation, and there's also MOTIS, developed by ISO, but these are practically equivalent. An OSI Layer 7 protocol.

A store-and-forward message-handling system (MHS) with graphics and fax capabilities. You know: attachments.

An EDI-specific protocol that is based on the X.400 protocol and which allows CAD/CAM, graphics, FAX, and other electronic information to accompany an EDI transaction.

The international standard for electronic directory services that store and serve (to clients using a variety of interfaces) centrally-managed personal location information.

Servers that interrogate X.500 directories can be found here. A bit more information is available on the German-language version.

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