Let X Be the Unknown

""A most ancient letter. Some say "chi", but the meaning is the same. Death - a letter that spells endings.""

- Master Xehanort, Kingdom Hearts

Naming something "X" makes it not only cooler, but more mysterious.

The fact that X represents a single unknown, generic value in mathematical equations may be partly responsible for this trope and is the source of the trope name.

See also Double X and Xtreme Kool Letterz.

Anime and Manga

 * X 1999
 * Racer X from Speed Racer.
 * In Death Note, Near literally lets X be the unknown, with "L-Kira" being the face of Kira that is pretending to be L, and "X-Kira" being the one who is doing the majority of the actual killings. It's not actually his name, but Near uses it like a name, it still uses X for mystery, and it still sounds cool.
 * Mr X, the Big Bad from Tiger Mask.
 * Ixpellia of the Lyrical Nanoha franchise, also known as the then mysterious "X/Ix/Ikusu" that the Mariage was looking for during the case in Striker S Sound Stage X.
 * GetBackers has the child prodigy Makubex, an orphan abandoned with the known name Makube and the unknown X.
 * In the Pokémon anime, "Pokémon X" is the Code Name for the Pokémon Lugia. On a related note, Pokémon X is also the name of a CD album that was released to commemorate the 10th anniversary of Pokémon.
 * Mysterious Girlfriend X, with the girlfriend in question being Mikoto Urabe.

Comic Books

 * Taken to the x-treme with X-Men. Almost everything has an X attached to it - the Man from Room X, Planet X, x-cetera.
 * "Weapon X" underwent a bit of a Retcon when the X was revealed to mean "10" in Roman numerals.
 * In an earlier comic it was supposed to be the Greek letter Chi.
 * Lest we forget, the leader of the X-Men, Professor X.
 * This is even invoked in a What If story where Magneto, instead of splitting with Xavier, decided to stay on his side and form the X-Men with him.
 * Element X powers Metron's Moebius chair.
 * Earth X in the Marvel Universe.
 * DC had an Earth X as well before the Crisis; it was the Freedom Fighters' world, where the Axis won World War II.
 * According to Word of God, this is because editorial nixed the idea of calling it Earth-swastika.
 * Buck Danny's nemesis is Lady X.
 * Mister X, one of Wolverine's enemies. And no, no one knows his real name. Yet.

Film

 * Science fiction/horror movie X the Unknown.
 * In The Great Escape, the resistance organisation is called the X organisation. Its leader is called Big X.
 * Both justified and Truth in Television: Security, obviously, was an issue, and calling something the 'X' Organization would give no clues to its purpose. Same concept was used in naming the three tunnels Tom, Dick, and Harry.
 * The kaiju film The X From Outer Space. Probably called that because the monster's actual name, Guilala, is silly as hell.
 * Monster X aka Keizer Ghidorah from the Godzilla films.
 * One of the spies in Chitty Chitty Bang Bang goes by the codename of X. His superiors get confused and think they are speaking to Rex or Tex.
 * There were at least two sci-fi films called Project X, one in 1968 and other in 1987.
 * Not to mention a recent comedy with the same name.
 * Low budget sci fi clunker The Man From Planet X.

Literature

 * Lampshaded and then subverted in Maskerade. Agnes Nitt wishes to be called 'Perdita X Dream' on the basis that X stands for 'a cool and interesting middle name'. The Lancre locals take it to mean she wants to be called 'Perditax'.
 * "Project X" is a top secret government project in Atlas Shrugged, which turns out to be a horrifying weapon of mass destruction based on certain frequencies of sound wave.
 * Discussed Trope in The Log by Craig Charles, in which he advises teams of adventurers confronted with a villain's huge library/database to go straight to the cabinet or folder marked "X". "Guys, no Mad Scientist ever named their Evil Plan to Take Over the World 'Project W'."
 * Nero Wolfe refers to a few villains as X.
 * Billy Collins' poem "The Names" is a tribute to the victims of 9/11. He lists one victim's name for each letter of the alphabet (Ackerman, Baxter, Calabro, and so on), but when he gets to X, he says that it will stand "for the ones unfound."

Live Action TV

 * In The X-Files, Mulder puts an X on the window to get in touch with his mysterious informant with no name or background (credited, appropriately, as Mr. X).
 * See also: the show title.
 * The name of Ted Mosby's college radio personality alias in How I Met Your Mother is "Doctor X".

Music

 * DMX.
 * Dierks Bentley's "How Am I Doin'" is credited to a "Writer X", who is actually Jim Beavers.

Professional Wrestling

 * TNA has the X Division, its official "no limits" division. It's unclear what the difference in rules is between an X Division contest and a heavyweight division contest, but in practice, it generally means a more highspot-oriented style.
 * The X-division is technically the TNA lightweight/middleweight division. It's called the X Division to draw attention away from weight classes, since the term "lightweight" has negative connotations of being weak or unimportant.
 * Short-lived wrestling promotion Wrestling Society X.
 * X-Pac
 * Wrestling Stable Degeneration X.
 * Er, that was actually DX taking the label - meaning the Degenerates of Generation X - and using it on themselves as a badge of pride. Sort of like N-Word Privileges, perhaps.

Radio

 * The early 1950s science fiction show Dimension X.
 * Also its mid 1950s revival, X Minus One.

Tabletop RPG

 * Paranoia: Vulture Warriors of Dimension X adventure series.

Toys

 * Mr. X, the Boogaloo doll by Blabla Kids seen as a supporting character in Kia Sorento commercials, "was designed to be a mysterious creature from a mysterious place who served as a secret friend to his young companions" according to a Blabla founder.

Video Games

 * Element X in Adventure Quest (an MMORPG).
 * Which has now been replaced by the considerably less awesome-sounding Harm and Void elements. The Guardian Dragon still claims to breathe Element X due to a healthy diet of Powerpuff Girls, though.
 * Dr. Wily once went by the alias "Mr. X" in Mega Man 6.
 * Mega Man X is a Black Box to everyone but his creator and has unknown limitless potential.
 * Mega Man ZX has the Guardian airship, located in...Area X.
 * Resident Evil has Mr. X, the name given to.
 * In the Avernum series, one of the archmages who govern magical use is named X. He's far more secretive and secluded than the other two.
 * In Condemned, there's "Serial Killer X," which... really doesn't make sense other than that the letter "X" sounds mysterious.
 * The X Parasites from Metroid Fusion.
 * Half Life: Opposing Force has its "Race X" aliens.
 * Streets of Rage has the main villain, Mr. X.
 * In Freedom Force, the basis of all power is the mysterious.... Energy X!
 * The Consonant Out Of Space?
 * Cave Story has Monster X, a Giant Space Flea From Nowhere boss.
 * The mysterious Mask X character from Punch Out!! spin-off Arm Wrestling, who turns out to be once unmasked.
 * Rumble Roses features Lady X (who is later revealed to be ) as the final opponent in each of the various wrestlers' story modes.
 * in Kingdom Hearts: Birth By Sleep. And to add confusion,
 * Every member of Organization XIII has a name that is a Significant Anagram of their original name with and X added in. Exceptions are  Interestingly the Japanese syllabary has no X, making their names "words that should not be pronouncable", a la Cthulhu.
 * Sector X is Iji is actually a subversion--although it seems mysterious at first, the actual reason it's titled Sector X is because it's the tenth sector, and X is the roman numeral for 10.
 * The villainous group in Paper Mario: The Thousand-Year Door are known as X-Nauts.
 * The X-Zone in Sonic Advance, and likewise XX in Sonic Advance 2. Both of them are the final level in their respective games...
 * Ace Attorney - Coldkiller X (Coldkiller Z in the original)
 * Sinclair's ZX80, ZX81 and ZX Spectrum home computers. Word of God is that the original machine was named after its Z80 processor, plus X for "the mystery ingredient".
 * Prior to its release, Pokémon Crystal, an Updated Rerelease of Pokémon Gold and Silver, was known as Pokémon X.
 * The Pokémon Lugia has also been known as "Pokémon X" in the past. Similarly, XD001 was the codename for the Shadow Lugia that is prominently featured in Pokémon XD.
 * The Japanese title of Super Smash Bros Brawl: Dairantō Smash Brothers X.
 * In the Visual Novel/Romance Game X Note, the titular Creepy Child (but not the main character) is called "X" by the researchers..

Web Comic

 * The webcomic Pokémon-X. The title has been hinted as being a reference to a creature or an item.

Web Original

 * Labtech X from Gaia Online.

Western Animation
"General Specific: I don't want an "Ex-Agent"! I want a current agent! Private Public: The "X" is put in to indicate mysteriousness rather than his job status sir. General Specific: Well why can't he choose another letter? Like "K"? Private Public: "K" isn't as mysterious a letter as "X", sir. General Specific: What about "L"? "L" is mysterious! [waving fingers] Llllllllllllll... Narrator: While the General re-acquaints himself with the alphabet..."
 * Brilliantly lampshaded in Sheep in The Big City, where General Specific has issues with the name of their latest hire, "X Agent":


 * Red X from Teen Titans.
 * Chemical X from Powerpuff Girls.
 * Planet X (see below for details) appears in many old sci-fi works. Today, most people recognize it from Duck Dodgers in the 24½th Century.
 * The Simpsons: Homer Simpson's online alias is Mr. X.
 * "So, Mr. X - if that is your real name..."
 * ' Mr X? Do I dare cross the final frontier?'
 * David Xanatos from Gargoyles. (yes, that Xanatos.) Sometimes he's even addressed as "Mr. X."
 * Protoform X (otherwise known as the murderous Rampage) from Beast Wars.
 * The Xs uses this trope.
 * Dimension X from Teenage Mutant Ninja Turtles.

Real Life

 * X-rays. They were discovered by Wilhelm Conrad Röntgen, but getting an X-ray sounds a lot snappier than getting your arm Röntgen'd, which you can actually do in Germany. Given that "Röntgen ray" is in line with a very long and rich tradition of naming inventions after the inventor, and that Röntgen himself (who was a German) didn't want them to be named after him, it is highly interesting that German-speaking nations (and Japan) followed the tradition, while English-speaking nations followed Röntgen's own suggestion.
 * Fun fact: according to the Other Wiki, Röntgen called them "X-rays" because they were an unknown form of radiation at the time. Now that X-rays are well understood, calling them that is a bit of a misnomer, but the name stays per the Rule of Cool.
 * The X-Prizes are something like this, originally the "X" being because a sponsor hadn't been found, but once people realized how cool the "X" sounded, it stayed, with the sponsor's name tacked on in front of it: "Google Lunar X-Prize", "Ansari X-Prize", etc.
 * Designer X, known also as Greg Kostikyan, RPG designer.
 * "Planet X" was the generic placeholder name for "whatever it is out there beyond the orbit of Neptune that's messing with its orbit". Pluto's discovery meant that "Planet X" now had the double meaning of "...beyond the orbit of Pluto" AND "the tenth planet". Of course, Science Marches On, and now not only have we discovered the Kupier Belt and the Oort cloud, but an entire class of Pluto-sized "dwarf planets".
 * For the record, the supposed irregularities in the orbits of Neptune (and also Uranus) were actually caused by incorrect calculations of Neptune's mass.
 * This is not the case with the X-Games; the first edition of the games in 1995 were named "The Extreme Games," making this a case of Xtreme Kool Letterz.
 * Conversely, consider Vince McMahon's short-lived American football league, the XFL. Viewers assumed that the name was an acronym for "eXtreme Football League", a reasonable assumption given the rash of Xtreme Kool Letterz in America during the late '90s. While this was the intent during early planning, XFL was the full name; McMahon himself once said that "If the National Football League stands for the 'No Fun League,' the XFL will be the extra fun league."
 * When ever the US Air Force has built a new aircraft that's still in the experimental stages there's always an "X" in it's title. When it is through the experimental stages and ready to be put into production the "X" is taken out and replaced with a classifying letter.
 * The Ecole Polytechnique, which is arguably the most prestigious french engineering school (college), is nicknamed "the X".
 * TV Tropes' Twitter: "Everyone, including paleontologists likes the letter 'X'. http://www.history.com/videos/predator-x-revealed#predator-x-revealed"
 * Malcolm X did this to his name. He was born Malcolm Little, but upon conversion to the Nation of Islam, he replaced "Little" with "X" on the reasoning that "Little" is a "slave name" (given to slaves and their descendants by slave owners), and "X" represents the unknown name lost in time. After he converted again, this time to Sunni Islam, he changed his name to El-Hajj Malik El-Shabazz. But he remains best known as Malcolm X.