Nom-de-guerre

When a soldier or agent has a name, but is instead routinely called by a nickname. Especially common amongst pilots for a variety of reasons. The Trope Name comes from The French, and translates to English as War Name.

There are various reasons why they might use a Nom-de-guerre. If they are members of some secret organization, or can't be entirely confident that their communications are secure, they might go by these alternate names as a way of preventing their identity being discovered by eavesdroppers. Or maybe many people share a name; this makes it clear who is being talked to or about. Being given such a nickname can be used to indicate inclusion into a circle of close friends or other group. Newbies to the organization might not rate a nickname, or else get saddled with generic names like New Guy #1 or New Guy #2.

Truth in Television for many organizations, particularly for aviation branches of the armed forces, where these nicknames are called "callsigns" and also aid in brevity of messages (depending on the equipment being used, if one guy is talking, nobody else can until he gets off the channel) and avoid confusion (in combat, or for that matter, in any relatively complicated flying operation such as formation flying or even just landing at a large enough airport, it is very important that everybody knows who is being told to fly at a certain course or to move in a certain direction).

Needless to say, if you are given a callsign and you don't like it, then you should keep in mind that The Complainer Is Always Wrong, and you can always get a worse one. Said nickname can be directly indicative of the person who has it, or it can be an Ironic Nickname, or it could refer to a Noodle Incident or some other noteworthy experience the person had with his friends.

Also serves as an In-Joke in many of these cases, making it clear to outsiders that there are things these guys know about each other that others don't.

Related to Known Only By Their Nickname and Code Name. Supertrope to Sobriquet, where the nickname in question is widely known due to the character's famed (or feared) effectiveness on the battlefield. Mustache De Plume is a variant of this that borders on Sweet Polly Oliver.

Comic Books

 * Any Superhero's name is essentially a Nom-de-guerre, and serves essentially the same purpose.

Fan Works

 * In the Mega Crossover Ranma and Akane: A Love Story, Ranma uses what she gladly admits is a nom-de-guerre ("Bushiko Ranma") when she moves to Nerima.

Film

 * Any military action movie featuring pilots is bound to have this, both because it is Truth in Television and because it's very cool.
 * Top Gun gives us Maverick, Iceman, Merlin, Goose, etc.
 * Flight of the Intruder has Cool Hand, Tiger, The Boxman, New Guy #1 Razor, Dookie, and Rabbit.
 * Iron Eagle doesn't actually feature very many actual fighter pilots in-plot, the only one we see much of is Chappie, whose callsign is a Shout-Out to General Daniel James, Jr., the first African American fighter pilot in the US Air Force.
 * This means, of course, that Chappie (in the film) likely got his call sign based entirely on being a black fighter pilot, though he'd have to necessarily be a very good fighter pilot to be named for Robin "The Wolf" Olds's second in command.
 * Red Tails gives us Lightning, Easy, Deacon, Junior, and numerous others.
 * Subverted in The Matrix. All of the Zionist rebels have such callsigns (Neo, Morpheus, Trinity, Lock, Switch, Tank, etc.), presumably based on their online handles before they were freed, in the case of people not born in Zion. It's a subversion because it's clear that they consider these their real names because they are the only names they have ever used in the real world.
 * Native Zionists, for their part, have real names that sound like they would be online handles, even though they have never been in the Matrix, due to lacking the modifications needed to interface with it.

Literature

 * The Black Company uses Meaningful Renames (usually either deliberately ironic or boringly descriptive—the most sadistically hardass sergeant in the company is named Mercy, while the mage missing an eye is called One-Eye) for much the same reasons as the French Foreign Legion. Wizards always go by nicknames as well, because letting people know your True Name (simply the name you were born under, in this setting) is tantamount to suicide.
 * Similar to above, the Malazan Marines from The Malazan Book of the Fallen acquire these when training or when first assigned to units, such as Hedge, Fiddler, Smiles and so forth.
 * The human members of the titular Monstrous Regiment all acquire noms de guerre, and are referred to by them for most of the book: Ozzer, Shufti, Wazzer, Lofty and Tonker. The vampire, troll and Igor are just Maladict, Carborundum and Igor, though. (Actually ).
 * Everyone in the Space Legion in Robert Asprin's Phule's Company series adopts a new name, which makes the Legion attractive to recruits wishing to hide from a shady past. In Phule's case, his real identity is publicly known.
 * And in Asprin's MYTH series, Guido comments when he joins the army that once you get saddled with a screwball nickname, you're stuck with it.
 * The Princess Bride: The Dread Pirate Roberts. Because The Dread Pirate just wouldn't carry the same sense of dread with it that makes him so effective as a pirate.
 * Into the Looking Glass Space Marines (and, later, those who work directly with them on ops) are assigned team names based on a trait, their name, or some event/activity they were involved in at some point (usually an embarrassing one).
 * In "Prospero Burns" (a novel in the Horus Heresy series from the Black Library) one of the Space Wolves is called simply "Bear". This is categorically not his real name, which comes back to save the day when a daemon that uses "the power of true names" to wipe the floor the the Space Wolves faces "Bear" who resoundingly beats the daemon. As a bonus for the long term fans, "Bear's" real name is "Bjorn", aka "Bjorn the Fell-Handed", the oldest living sentient being in the Imperium (being over ten thousand years old by the time of "now", the year 40,000).
 * During the Yuuzhan Vong war, Jaina Solo joined Rogue Squadron and gained the callsign "Sticks". (Stick #1 was her X-Wing's control stick. Stick #2 was her lightsaber.)

Live-Action TV

 * The pilots of the 2004 reboot of Battlestar Galactica are known largely by their call signs, especially when they aren't a main character. For example, Kara Thrace is as likely to be called "Kara" as "Starbuck," but "Crashdown," "Kat," "Racetrack" and other minor characters were identified almost solely by their call signs. This is a more realistic take on how the original show did it in the 70s, where everybody appeared to only have callsigns.
 * JAG: Captain Harmon Rabb, during his combination Ten-Minute Retirement and Back in the Saddle, usually went by "Pappy" during his time serving aboard the USS Patrick Henry, because he was older than the other pilots. Eventually, the men in his squadron changed his callsign to "Hammer", after his father, a fighter pilot who went MIA during the Vietnam War, because they knew his dad would be proud of his achievements.
 * Of course, in one episode, when a bad guy in a hospital demands that he identify himself, he claims his callsign is "Zapper". And then zaps the guy with a defibrillator.
 * Though he didn't use it in the series, Stargate SG-1's Cameron Mitchell used the callsign "Shaft" during his time as an Air Force pilot.

Professional Wrestling

 * Hunter Hearst Helmsley, known to most as Triple H (or "Trip").

Video Games

 * The Wing Commander games, books, cartoon, and movie give us quite a few, including Maverick, Iceman, Angel, Maniac, Spirit, Paladin, Doomsday, Jazz, Bear, Hunter, Vaquero, Vagabond, Hobbes, Flint, Hawk, Seether, Catscratch, Deathstroke, Starkiller, etc.
 * In one of the books, Maverick learns that Maniac was passed over (again) for promotion to Lieutenant Colonel. The narration notes that Maverick quietly agrees with the higher-ups that it was rarely good for a pilot's callsign to directly indicate his state of mind.
 * Many of the Jagged Alliance mercenaries have callsigns, rendered like "Carl 'Reaper' Sheppards" or "Kyle 'Shadow' Simmons". If the mercs ever have lines that refer to other mercs, the callsigns are almost exclusively used in favor of the given names.
 * Used for many of the assassins in the first No More Heroes, with a few exceptions. Averted for the most part in No More Heroes 2: Desperate Struggle, where they mostly go by their real names.
 * Many of the troopers in Task Force 141 in Call of Duty: Modern Warfare 2 use callsigns such as Ghost, Roach, and Worm. Inverted, in that Captain MacTavish's nickname is "Soap", but the first time anyone calls him that, the nearest trooper asks "Who's Soap?"
 * In Modern Warfare 3, Sandman, Grinch, and Truck of Metal team are only known by their callsigns. Derek "Frost" Westbrook is the only one whose full name is given, and
 * In the 2010 Medal of Honor, the members of AFO Wolfpack and AFO Neptune go exclusively by their callsigns, even when introducing themselves to fellow American soldiers such as Sgt. Patterson.
 * Also, several minor characters are mostly referred to by their callsigns, mainly because they are pilots or aircrew in aircraft. Reaper 31 is the callsign for an AC-130 gunship, and two Apache helicopter crews go by Gunslinger 6 and Gunslinger 11.
 * Jeff Moreau a.k.a. Joker in the Mass Effect series received his nickname in flight school from an instructor as an Ironic Nickname since he was a grim young man due to the disorder that made his bones incredibly brittle. Perhaps the first time he smiled was when he graduated top of his class and became the snarky Ace Pilot that we know and love.
 * The Metal Gear series has this for many player characters and bosses, often times not learning their real names. Notably, different units have different naming conventions. The Cobra Unit are named for the emotion they bring into battle (i.e. The Fury, a guy with a flamethrower), FOXHOUND are named for an aspect of themselves plus an animal (i.e. Vulcan Raven, who is an inuit shaman with a minigun), and Dead Cell uses nicknames in reference to their profession (i.e. Fatman, a Mad Bomber). The B&B Corps use the emotions of the Cobras and the animal name of FOXHOUND, while using the weapons of Dead Cell (i.e. Raging Raven, who wields a grenade launcher and a flight harness).
 * Named Ace Combat pilots usually have callsigns, either "[Squadron Name] + [Number]" (e.g., Scarface One, Mobius One, Wardog Three) or a unique nickname (e.g., Edge, Pixie). This also applies to enemy pilots. Especially badass pilots get special names like "Demon Lord."
 * Lost Planet 2 implements these as badges the players show under their names, such things range from simple "Machine Gun Expert" to tongue-in-cheek jokes regarding the game itself.

Web Comics

 * Callsigns are serious business in Air Force Blues, since it's nominally set in the US Air Force. There's a special ceremony held to confirm main character Ken Dahl as "Barbie" and choose one for Lt. Willows.

Western Animation

 * In G.I. Joe both the Joes and Cobra agents usually only go by their code names.
 * Robot Chicken: "The name's Fumbles. It was always Fumbles."

Real Life

 * Common in the military, particularly amongst pilots, who use their nicknames as "Callsigns", basically a distinct way of identifying themselves over the radio without using their names. Not only does it help protect your identity from the enemy, but it also helps avoid confusion when your squadron includes three Smiths, five Johnsons, and a couple of Sanchezes.
 * One use of the Trope in a non-warfare context is when writers, artists, or performers use Stage Names or Writing Names (the latter also known as a Nom De Plume (see also Moustache De Plume for female authors using male names).
 * Edda van Heemstra, a ballet dancer who supported the Dutch Resistance during World War II. Now better known as Audrey Hepburn.
 * Practiced by prominent members of the French Army during the ancient regime and still widely practiced by Foreign Legionaries. In World War Two, prominent leaders of the Free French (and about every member of La Résistance) adopted Noms De Guerre to protect their families from retribution from the Nazis and Les Collaborateurs.
 * Such as Darius Paul Bloch, who used Dassault as his alias. His brother took the name as well and used it for his aviation company.
 * Saloth Sar, better known to the world as Pol Pot. He took that name to lead his guerilla movement, the Khmer Rouge.