"A most ancient letter. Some say "chi," but the meaning is the same. Death - a letter that spells endings."Naming something "X" makes it not only cooler, but more mysterious. "X" is often associated with anonymous persons and things that have yet to be formally named. 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.
— Master Xehanort, Kingdom Hearts: Birth by Sleep
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Anime and Manga
- Racer X from Speed Racer. Played straight with respect to the other characters, but inverted with regard to the audience, since the narrator won't stop reminding you just exactly who he is.
- 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 StrikerS Sound Stage X.
- Get Backers 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 Xnote 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.
- 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.
- 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. In that case, "X" is short for "Xavier", his actual name. Once, a flashback sequence has Moira MacTaggart point out that naming the team after himself was a bit egotistical. Xavier insists that the connection never occurred to 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.
- Captain America and The Avengers foe Baron Zemo (the original) couldn't remove his hood because of a Freak Lab Accident involving Adhesive-X. Admit it, "X" even makes "I'm angry because this mask is glued to my face" cool.
- 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 Godzilla Final Wars.
- 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. It doesn't help when the other spy tries to clarify that it's X as in "X and bacon."
- There are at least three films called Project X: one in 1968, on in 1987, and the 2012 comedy.
- Low budget sci fi clunker The Man From Planet X.
- Star Trek III: The Search for Spock began a tradition in the franchise where experimental starships showcasing either cutting edge ship design or technology were christen with NX rather than the typical NCC. And later rechristened with NCC when it no longer was a prototype design (the Excelsior was NX-2000 when it showed up in The Search for Spock. When it showed up again in Star Trek VI: The Undiscovered Country, in-universe eight years later, the Excelsior was NCC-2000).
- Lampshaded and then subverted in Maskerade. Agnes Nitt wishes to be called 'Perdita X Dream' on the basis that X stands for 'someone with a cool and interesting middle name'. The Lancre locals gently mock her for it, calling her 'That Agnes who 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."
- In Darkness at Noon, Rubashov invokes this, charging the Party has lost touch with the masses, which have become once again "the great silent x of history":
"A mathematician once said that algebra was the science for lazy people—one does not work out x, but operates with it as if one knew it. In our case, x stands for the anonymous masses , the people. Politics means operating with this x without worrying about its actual nature. Making history is to recognize x for what it stands for in the equation."
- In The Diamond Age, the local crime boss can't tolerate how westerners mangle the pronunciation of his name, preferring they just use the first letter of its Pinyin transliteration. Hence, "Dr. X". Eventually even the local judge starts calling him that.
- In the introduction to the 50th anniversary reprint of the Doctor Who Expanded Universe novel Prisoner of the Daleks, Trevor Baxendale says that he knew he wanted the Dalek Inquisitor-General to have a name, but until he could work out the right name, he referred to it as "Dalek X". Eventually he realised the right name was obvious: Dalek X.
Live Action TV
- The X-Files
- The title refers to the mysterious files regarding the supernatural that the heroes investigate. It's later revealed that these files are put into the "X" drawer because there's no specific place to put them and it has the most room.
- 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).
- The name of Ted Mosby's college radio personality alias in How I Met Your Mother is "Doctor X".
- Mutant X, in which "Mutant X" is an underground cell of mutants protecting mutants.
- The game show What's My Line?, where a panel had to guess the occupations of the contestants, sometimes included contestants whose names were well-known but whose faces weren't, who would be introduced to the panel as "Mr. X". (For example, Jacques Cousteau was Mr. X in one episode.)
- A CSI: New York arc had the team chasing a hitwoman they referred to as "Suspect X"; they only learn her name in the last episode she's in.
- In the Star Trek franchise, experimental starships are given an "NX" designation until the spaceframe is approved for mass construction and an "NCC" registry.
- Numerous masked jobbers named "Mr. X" or "Dr. X".
- TNA has the X Division, its official "no limits" division, as opposed to its heavyweight division which had a minimum weight limit. It's essentially a hardcore division but not in the Garbage Wrestler sense, in that more "high spots" are to be expected sense. Despite having competitors approaching 300 lbs and a celebrated champion in Samoa Joe, a weight limit was eventually put in place for those who wanted to compete in the division, temporarily turning it into a glorified cruiser weight division until it was lifted.
- Short-lived wrestling promotion Wrestling Society X, which took place in a secret underground X bunker.
- D-Generation X. Shawn Michaels and Triple H took the label Degenerates of Generation X and used it on themselves as a badge of pride. Sort of like N-Word Privileges, perhaps. X Pac from the same group.
- Gadget X, the scientists' untested rocket in The Golden Apple.
- 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.
- Element X in AdventureQuest (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 The Powerpuff Girls, though.
- Mega Man
- Resident Evil 2 has Mr. X, the name given to the Tyrant that stalks you.
- 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: Criminal Origins, 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 Bald Bull once unmasked.
- Rumble Roses features Lady X (who is later revealed to be Reiko and Fujiko's Missing Mom turned into a cyborg) as the final opponent in each of the various wrestlers' story modes.
- Kingdom Hearts:
- The series' overall Big Bad is Xehanort, who has something of an obsession with the letter 'x'.
- Every member of Organization XIII has a name that is a Significant Anagram of their original name with an X added in. The exceptions are Xemnas and Xion, the former because he took someone else's identity and the latter because she isn't a Nobody and was originally created as a Sora replica. Interestingly the Japanese syllabary has no X, making their names "words that should not be pronouncable", a la Cthulhu.
- The χ-Blade, the ultimate Keyblade in Kingdom Hearts: Birth by Sleep. And to add confusion, it's based on the Greek letter "Chi," meaning it's pronounced exactly like "Keyblade." It can get a little awkward when using verbal communication.
- As of 3D, it turns out that Xehanort used the X as a symbol for potential bodies in his massive Grand Theft Me plan. He calls it the "Recusant's Sigil", and considers anyone marked with it his property. He can even track people who wear it on their clothes - such as Sora's dream-world outfit.
- 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... unless you're playing as Sonic and have all the Chaos Emeralds.
- 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 Re-release of Pokémon Gold and Silver, was known as Pokémon X.
- And now, of course, Pokémon X is actually a thing, with the latest (6th) generation being X and Y.
- 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 Romance Game X Note, the titular Creepy Child (but not the main character) is called "X" by the researchers as a direct invocation of this trope, since he refuses to give his name and does actually represent the unknown. He turns out to be Anon. The sequel Area X follows this tradition, taking its title from the dimension Area-X, aka the 'unknown' dimension.
- In Persona 4, Chie and Yukiko made a curry-like dish so inedible that there's no way to call it curry. Yosuke dubs it Concoction X or Mystery Food X
- Dance Dance Revolution X and X2.
- In Psychic Storm, "Stage X" ensues when the player encounters an unidentified Giant Space Flea from Nowhere.
- The Doom mod Scythe 2 includes a level named "Mr. X", where the player fights his own Evil Counterpart.
- Pokémon-X. The title has been hinted as being a reference to a creature or an item.
- Lampshaded in Sheep in the Big City, where General Specific has issues with the name of their latest hire, "X Agent":
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...
- Red X from Teen Titans. It was initially a phony supervillain identity Robin used in an attempt to get close to Slade, but someone else came along and stole the gear for himself. His identity is a total mystery.
- Chemical X from The 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 24˝th Century, which parodies this by having it as an actual planet with a giant "X" on it, which you can find by following a trail of planets similarly labelled with other letters of the alphabet.
- 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?'
- "So, Mr. X - if that is your real name..."
- 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 X's uses this trope.
- Dimension X from Teenage Mutant Ninja Turtles.
- Alien X, originally presented as the mysterious tenth alien of Ben 10: Alien Force.
- The variable "X" is almost always used as an unknown in mathematics starting with algebra, by convention. You'll get more of the alphabet in problems with many unknowns, but "x" is pretty much always the first one.
- 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.
- 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 Costikyan, 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 almost always always an "X" in its designation. 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 (i.e. the Lockheed Martin X-35 became the F-35 Lightning II). The other branches do the same with their experimental weapons systems, though slightly differently (the classifying letter is usually already part of its name; for instance, the Heckler & Koch XM8 would have become just the M8, had it been adopted). This association is so pronounced that experimental aircraft (especially ones designed for the US military) are often known simply as "X-planes." The Air Force used to do that too, such as in the case of the XB-52. (And the Army Air Force before that, such as with the XP-51.) What was done with the F-35 is just weird, considerig the "F" ("fighter") series is only up to F-23.
- Note that the X- strictly speaking applies to experimental aircraft. Once they are on the path to finalizing the design for production, it is common to switch to a Y-prefix. Compare the Lockheed XP-38, a one-of-a-kind prototype built to see what the design could do, and the YP-38, a pair of pre-production models built using lessons learned from the original.
- 'X + number' alone denotes the experimental aircraft, but prototypes are designated in the 'XF, XB + number' style - second letter designating purpose - fighter, bomber etc., then moving to the 'YF + number style', when the design is in the pre-production phase. F-35 is something of an odd number out, as instead of assigning new number in the fighters sequence to it, it retained its '35' numerical designation, with the original prefix 'X' just changed to 'F'.
- The Ecole Polytechnique, which is arguably the most prestigious French engineering school (college), is nicknamed "the X".
- 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.
- Microsoft's (mostly) video game API DirectX. X is taken as the variable for the APIs within the API (Direct2D, Direct3D, DirectMusic, DirectInput, etc.)
- The XYZ Affair.
- Before Pliosaurus funkei was formally described in 2012 it was referred to in the paleontological community as Predator X. As TV Tropes' Twitter put it: "Everyone, including paleontologists likes the letter 'X'."