This is a fairly simple trope. It is simply the habit of calling Earth "Terra" in Sci Fi. The word is adopted from the Latin word for, well, earth.
It is used to make the planet Earth follow the Roman naming systems for the planets of the Solar System and also because "Terrans" is a more respectable description for the inhabitants of the planet than "Earthlings". Another advantage is that it is language-neutral, since it is by far the most common word for the planet—four world languages call this planet Terra or some variant thereof,note as do many of the other Romance (i.e. Latin-derived) languages with fewer speakers.
Also, Terra provides a convenient standardization in that Earth is otherwise one of the only two planets in the Solar System that are not named for Roman deities, with Uranus, named for a Greek one, as the other, and one of the few things in general in the Solar System that are not named for Roman or Greek mythology, along with a few others, such as the likely dwarf planet Makemake, named for the creator in Easter Island folklore, or the Uranian moon Puck, named for a character in Shakespeares A Midsummer Nights Dream.
This is often accompanied by reference to Earth's sun as "Sol", and the moon as "Luna", to differentiate them from other suns or moons.
Calling Earth "Terra" may be a result of an Earth That Was scenario.
Due to the evolution of the pronunciation of Latin itself, the older form "Tella" also fits here. SF authors of the 1930s also applied an alternative version of the word, "Tellus," which means the same thing.
For whatever reason "Terran" is frequently used as an alternate name for humans even when the planet is still called Earth.
- In Trinity Blood, Methuselahs refer to humans as Terrans. Because, as the original novels reveal, the first Methuselahs were created on Mars.
- Toward the Terra.
- In Outlaw Star, humans are mostly referred to as just that, but some characters refer to them as Terrans. Notably Aisha, and usually with a note of derision.
- In Space Battleship Yamato 2199, the alien Gamilas call Earth Terron which is explained as their rendering of Terra/Terrans. (Apparently they couldn't tell the name of the planet and that of the species apart—they cannot do that for their own planet, either.)
- Nemesis the Warlock: Far future Earth is referred to as both "Terra" and "Termight", the latter being a bastardization of "termite". This is because the planet's surface has become inhospitable and humanity has started living underneath the planet's surface in massive city-caverns, connected to each other through a super-fast highway system and to the outside universe through a wormhole.
- The primary kaiju home world and setting for the Godzilla series in The Bridge is referred to as Terra, which is also a moniker for certain labels. As such the benign kaiju faction is referred to as the Terran Defenders to distinguish them from the neutral or malign Mutations.
- In Rocketship Voyager humans still refer to their homeworld as Earth, but Terra is the more formal name used with "extraterran" races.
"State your destination," he said.
"The next groundside transport is scheduled for__"
"The planet Earth! Terra, third planet of the Solar System."
- In the Marvel Cinematic Universe, almost all alien races refer to humans almost exclusively as Terrans, and to Earth as Terra or the Terran Homeworld. However, Rocket has also been known to call Peter Quill or Scott Lang a "humie", so they're at least aware of the word humans use for themselves. This has its basis in the original comic books.
- In the Catteni series by Anne McCaffrey, the humans are referred to as "Terrans" by the alien Catteni, and sometimes by the humans themselves.
- In Sergey Lukyanenko's Line of Delirium trilogy, Earth is renamed Terra and serves as the capital of the Human Empire. It appears to have happened sometime during the Vague War, as a note Kay finds dating back to the war still refers to the planet as Earth.
- In the novelization of Men in Black, humans are called "terries". (At least by Edgar the bug, so it's likely a derogatory derivative of "terran".)
- "Terra" is the most common name for the home planet of the titular character in Perry Rhodan.
- Keith Laumer's Retief stories frequently use the abbreviated "Terry" for Terrestrial. There's no sense of it being derogatory, as the Terries use it themselves often enough. Also, Earth's diplomatic corps is known as the Corps Diplomatique Terrestrienne.
- In Francis Carsac's Robinsons of the Cosmos, the Transplanted Humans end up calling their new planet "Tellus". At the end, they name their new unified nation the Union of Tellus Republics.
- E. E. Doc Smith's stories referred to our planet as both Earth and Terra. He also used the alternate name Tellus.
- The Space Trilogy: Aliens and space-travelers in Perelandra and That Hideous Strength occasionally refer to Earth as "Tellus," the Latin name for Earth.
- Earth has become known as "Terra" in S.L. Viehl's Stardoc series.
- An odd inversion is that Edmond Hamilton's Starwolf trilogy always speaks of humans as Earthmen, but the back-cover blurb describes the main character as a Terran.
- In Elliot S. Maggin's Superman novel Last Son of Krypton, Lex Luthor complains when aliens call him an Earthling, because he prefers "Terran". They explain that the translator operates according to the listener's intention, so if Luthor decides he wants to hear "Terran" instead, that's what he'll hear. (It works, at which point he decides to make the translations of his interrogators' names and species terms as insulting to them as possible.)
- Poul Anderson's Technic History stories refer to Terra and the Terran (or sometimes Terrestrial) Empire. Anderson justifies the use in an odd way: once, an alien asked where he comes from replies, "from earth" - but that is what the name of his home planet means if he translates it, so "Terra" is used to distinguish various earths spread throughout the Galaxy.
- In Bob Shaw's Who Goes Here? many of the officers of Earth's military prefer the term "Terra". One enlisted man notes it's often a sign of an idealistic fool who's likely to get his men killed; the ones who say "The Mighty Terra" are particularly bad. Naturally, the protagonist's commanding officer is of this type.
- Bob Carrau's Monsters and Aliens from George Lucas refers to Earth as "Urthha".
- In The Expanse Earth is still referred to as Earth but the Moon is called Luna to distinguish it from the numerous other moons colonized by mankind. The Sun starts being referred to as "Sol" in later books when mankind expands to different solar systems through the gates.
- Isaac Asimov's "Mother Earth": There's Earth and Earthmen, but it is a Terrestrian government, and Terrestrian resources.
- Creator/N.K.Jemison's "Emergency Skin" follows a man who goes to Earth to get skin cells for de-aging and skin replacement procedures, and is surprised by "Tellus'" diversity and acceptance.
- While planet Earth is never explicitly named "Terra" in Chrysalis, the sentient Von Neumann probe containing the last human consciousness is referred to by aliens as "the Terran", much to the Terran's dismay. Ultimately, though, the race of sentient robots they build out of procedurally-generated human brain scans wind up inheriting their alien moniker, mostly out of the need to differentiate themselves from the long-extinct biological humans.
- Star Trek:
- The Mirror Universe has the Terran Empire (until it collapses, at least). Interestingly enough in the Star Trek: The Original Series episode "Mirror, Mirror" which is the first in the franchise (and only on that series) in showing the Mirror Universe the term "terran" is never used and Earth's empire is just called The Empire. The term Terran and Terran Empire comes from the Deep Space Nine episode and its first chronological use was in DS9's episode "Crossover". Later the term was use retroactively in the two prequel series Star Trek: Enterprise and Star Trek: Discovery.
- The main universe occasionally also refers to our solar system as the Terran System (though it's more frequently known as the Sol system or Sector 001).
- Spock is asked in one Original Series episode if he is "Terran or Vulcan."
- In Star Trek: Enterprise, there is a human supremacist group named "Terra Prime". There is also a failed colony known as Terra Nova.
- The Deep Space Nine episode "Valiant" uses the "calling the moon 'Luna'" variant. Jake thinks his grandfather (who lives on Earth) is old-fashioned for calling it "the moon, like it's the only one or something," but apparently nobody who lives there calls the moon "Luna" either.
- Blake's 7 has a Terran Federation that serves as the antagonist to the ragtag bunch of anti-heroes. There's also a crime syndicate that calls itself Terra Nostra.
- In Stargate SG-1 it was mentioned once that the Ancients called Earth, Terra. Of course, Latin is derived from their language. Though most of the time the Goa'uld word, Tau'ri, is used for humans from earth. "Tau'ri" (lit. "The First World") is the name for Earth itself. Humans from Earth were originally referred to as "Humans of the Tau'ri," and was eventually shortened to just "Tau-ri" for the sake of brevity.
- Played with and finally averted in the original Battlestar Galactica—"Terra" turns out not to be Earth, but rather a splinter group from the Thirteenth Tribe that colonized Earth. Surprisingly, their history is similar to ours, except on Terra Those Wacky Nazis and Dirty Communists have joined togethernote to form the Eastern Alliance, a totalitarian regime that seeks to rule the universe. They also sign peace treaties that they have no intention of keeping (Does This Remind You of Anything?).
- Doctor Who stories written by Robert Holmes occasionally have aliens referring to humans as "Tellurians" as a Creator Thumbprint, most overtly in "Carnival of Monsters" and "The Two Doctors".
- Space Patrol (US) has Terra, a sort of Space Canberra that functions as both the capital city and base of operations for the Space Patrol.
- The BattleTech universe had the Terran Hegemony. The planet is mostly referred to as Terra, as well.
- Renegade Legion has an imperial government known as the Terran Overlord Government.
- Holy Terra is the homeworld of the Imperium of Man in the Warhammer 40,000 universe.
- The use of the Latin word is particularly appropriate, as 40k has a translation convention of using Latin (or debased Latin-like words) to represent "High Gothic"—the archaic administrative and religious language of the Imperium of Man. That said, even when characters are using the more colloquial "Low Gothic" (represented by English, essentially the descendant of all human languages mish-mashed together with, by the time of the settings present day, 22 thousand years of linguistic drift, 5 thousand year FTL collapse and horrid strife and 10 thousand more years of linguistic drift) they still tend to call the human homeworld "Terra". The term probably filtered down into Low Gothic from High Gothic, given that to the vast majority of Imperial citizens Terra is a near-mythical place of superlative religious significance, referred to mainly in hymns and prayers.
- The Horus Heresy series shows that this isn't an all-that-old development, given the time scale of the world - by the times of Heresy, two centuries after the inception of the Imperium, some Astartes still refer to the Throneworld as Earth. The Beast Arises, which takes place some two thousand years after, also notes that "Earth" is still one of the names for Terra, although by that time it's clearly going out of use but ten thousand years after the Horus Heresy, in the 41st millenium (40999 Gregorian), ancient Adeptus Administratum records do recognize it as the first common name the planet was referred to by ancient humans before first political unification.
- The Starfire universe has the Terran Federation.
- The Terran Confederation in Traveller.
- Terrans are alternately called "Solomani"(men from the sun) and that is the preferred rendering in the Third Imperium era which is the default time of the GURPS line.
- Inverted in Manhunter, the long out-of-print sourcebook for Rifts (and the only officially-sanctioned Rifts book not published by Palladium Books). The book has Earth as well as Terra, the later is a new homeworld founded by Humans after Earth was rendered nearly uninhabitable.
- Zig-Zagged in Twilight Imperium, the "human" faction is known as the Federation of Sol, but the home planet itself is called "Jord", "soil" in the Scandinavian languages.
- In X3 Terran Conflict, the humans in the Sol system are referred to as "Terrans", but the planet is still called Earth, as is their government (the Earth State); the games make the distinction between "human" used to refer to the species as a whole, and national affiliation like the Terran, Argon, and Solaran. In X Rebirth, the residents of the Republic of Cantera are sometimes referred to as Terrans, being an Earth State Lost Colony.
- Happens partially in Star Control, where instead of the Sun and the Moon we have Sol and Luna. Earth is named Earth and the humans Earthlings and not Terrans, however.
- The Galactic Terran Aliance from FreeSpace. Earth is still Earth, however.
- In the sequel, two sides use Terran in their name: the Galactic Terran-Vasudan Alliance (composed of humans and Vasudans) and the Neo-Terran Front (composed of anti-Vasudan human radicals, with a vision of 'Neo-Terra' as an utopia for humans).
- In StarCraft, humans who aren't from Earth are called Terrans. Humans from Earth are called...humans. Earth is still called Earth.
- The French translation uses untranslated "Terran" for the Terran confederacy (and later empire) of the Koprulu sector, and translated "Terriens" for the United Earth Directory forces.
- Wing Commander has the Terran Confederation, with "Terrans" occasionally being used by non-human races to refer to the speciess originating from Sol III. The Expanded Universe makes it clear that the Confederation is centered on Terra/Earth, but includes members from other species, such as the Firekkans.
- In the Unrealverse, the Skaarj call humans Terrans.
- PlanetSide has the Terran Republic, which still calls its home Earth.
- The main race in the sequel to Galactic Civilizations is the Terran Alliance, though it can be renamed.
- Rock n' Roll Racing refers to Snake Sanders' home planet (very blatantly Earth-like) as Terra.
- In Final Fantasy IX the plot's impetus is an invasion by another planet known as Terra. The homeworld is Gaia (Greek for "Earth").
- The Battle Opera Gunmech universe has the Terran Alliance.
- Used in Crash Nitro Kart. Earth and Terra are separate planets, and one of the inhabitants of Terra believes Earth to be a copy of it.
- In Mass Effect, the Star of Terra is one of the Systems Alliance's highest military decorations. If you picked the War Hero background, Shepard was awarded it after the Skyllian Blitz.
- Conquest: Frontier Wars has humans called Terrans, but Earth is still Earth.
- The leader of the planet-themed Stardroids in Mega Man V is named "Terra", however in the Japanese version he's simply known as "Earth".
- Subverted in Earth & Beyond. Earth is still called Earth, although humans who live on its surface (and those who owe allegiance to Earth) are called Terrans.
- Subverted in Star Citizen. There's a planet called Terra, but it's actually a major colony that happens to look extremely similar to Earth. Earth itself remains and is still the political, economical and military core of the titular United Empire of Earth(UEE).
- Played with in Star Ruler and Star Ruler 2, where the human faction are referred to as "Terrakin". The plotless first game never explains why the starting planet isn't Earth/Terra, but the second game's backstory explains the playable Terrakin as a Lost Colony whose colony ship was thrown way off course.
- Played with in Master of Orion: Conquer the Stars, where the Terrans are Transplanted Humans hailing from Alpha Ceti (where they were dumped by the Antarans after they were no longer needed). The main humanity is still called "Human". The Terran Khanate is the Evil Counterpart to the Human Republic.
- Stellaris allows you to name your starter planet, but the generic human template starts on Terra in the Sol system.
- In Schlock Mercenary Earth still retains its English name. But the star it orbits is known as "Sol", and its terraformed and heavily populated moon was renamed "Luna" by "The Committee for Differentiating Our Planet From a Bare Behind." The term "Terran" is in use, but it refers to any of the many sentient races hailing from Earth (thanks to widespread use of uplifting) and not just humans.
- Escape from Terra, obviously. And many of the planet's inhabitants are referred to as "Terries".
- All beings that can trace their lineage back to Earth in Orion's Arm are referred to as 'Terragen', and the region of space they inhabit is, obviously, called the Terragen Sphere. Note that most terragens were not actually born/created on Earth, and are hence not Terrans proper—they're just all, ultimately, descended from or created by Terrans.
- Often used as part of Memetic Mutation tumblr scifi posts examining the complexities of Earth life in general and humans in particular; the subgenre is alternatively known as Space Australia, Humans Are Space Orcs, and Humans Are Weird.
- Battle for Terra is a subversion. Terra here is an alien planet named so by the humans fleeing the destroyed Earth. Its real name is unknown.
- The baseline humans are referred as "Terrans" in Exo Squad to distinguish them from Neosapiens and get around the small fact that both races are human, regardless of the fact that they don't call Earth "Terra", or that many humans are natives of Venus or the moons of the outer planets.
- One of Perry Rhodan's authors, Willi Voltz, used the word in a political context. He was heard addressing his readers as "Terrans" and explaining that he wanted to have this understood as a honorific of people whose mindset was advanced enough that they would identify themselves as members of the human species as a whole and not of an ethnic or national community.
- Though Terra is seldom used in English, the derivative terrestrial is commonly used as an adjective for Earthly things. It can also describe something pertaining to land as opposed to sea, since Terra can also mean soil or dirt.