Terran was the term consistently used to refer to Humans from the mirror universe. (DS9: "Crossover")
History[]
On April 5, 2063, a group of Terrans were filled with fear and suspicion when a Vulcan scout ship landed on Terra to initiate first contact. The Terrans slaughtered the Vulcan crew and used the technology within the scout ship to create the interstellar Terran Empire.
The Terrans conquered many of the races they encountered, including the Vulcans, Orions, and Tellarites. as well as the Denobulans and Andorians. Many species became slaves of the Terrans, but some species, like the Vulcans, were treated better, and could even serve in the Empire's military. (ENT: "In a Mirror, Darkly")
This interstellar empire lasted almost three centuries. In the year 2267, Spock encountered the prime universe James T. Kirk, and became determined to change the empire. Spock eventually became the empire's ruler, and instituted reforms that made the empire much more peaceful. (TOS: "Mirror, Mirror") However, these reforms led to the Terran Empire's downfall; they resulted in the empire's military strength being unable to prevent the equally vicious aspirations of the Alliance of Klingons, Bajorans, and Cardassians from conquering the Empire. When their empire fell, the Terrans were forced into slavery by the Alliance, along with the Vulcans. (DS9: "Crossover", "Through the Looking Glass")
At some point during this time, the Terran High Chancellor was killed after his attempts to reform the empire failed. The crew of the ISS Enterprise mutinied and fled to the prime universe with refugees seeking a new life as a result. At least most of the crew ultimately survived and found a new life amongst the United Federation of Planets. (DIS: "Mirrors")
In the year 2371, after an encounter with several prime universe individuals, Benjamin Sisko, a Terran pirate working for the Alliance, decided that he had had enough of slavery and began a revolt. He led a number of Terrans and other species in the Terran Rebellion, a revolution against the Alliance. Though Sisko was killed the same year, the rebellion still lived on. (DS9: "Crossover", "Through the Looking Glass")
A year later, in 2372, the rebels captured the space station Terok Nor above the planet Bajor. The Alliance's ruler, Regent Worf, led a fleet to retake the station, but the rebels managed to drive the Alliance fleet off. (DS9: "Shattered Mirror")
Two years later, in 2375, the rebels captured Worf himself. When last seen, the rebels appeared to be well on their way to achieving victory over the Alliance and winning back their freedom. (DS9: "The Emperor's New Cloak")
By 2385, the Terran Empire was reformed. Chakotay, Dal R'El, Jankom Pog and Murf encountered the Terran crew of the ISS Voyager-A when they were shifting to different realities due to a temperal fracture in spacetime. (PRO: "Cracked Mirror")
During the 27th century, the mirror universe and the prime universe moved so far apart from each that further crossings between the universes became impossible. The Federation therefore had no way of knowing what became of the Terrans after this date. (DIS: "Die Trying")
Biology[]
"Only compared to a Human from your universe. It's the singular biological difference between our two races."
Terrans and prime universe Humans were physiologically almost identical, since they were exactly the same species, just hailing from different universes. The only immediately noticeable difference was cultural, and even then, it was possible for a prime universe Human to feign bigotry and barbarism and, though more difficult (TOS: "Mirror, Mirror"), a Terran to feign tolerance and decency, meaning an individual could blend in until they were able to return to their home universe. However, there was one physiological difference: Terrans were more sensitive to bright light. This fact led Michael Burnham, upon discovering it, to quickly realize that the Gabriel Lorca she had been serving under had been his mirror universe counterpart all along. (DIS: "Vaulting Ambition")
Although this was only revealed in "Vaulting Ambition", in mirror universe episodes predating Discovery and following "Mirror, Mirror", Terran starships and habitats are noticeably darker than a Federation starship would be, and Jonathan Archer appeared to squint while in the captain's chair of the USS Defiant. As such, "Mirror, Mirror" not featuring dimmed lights on the ISS Enterprise was likely a product of the mirror universe being a one-shot location that the production crew did not expect to be revisited in the later productions that fleshed it out - in other words, what TV Tropes would call "Early-Installment Weirdness".
In "The Wolf Inside", Michael Burnham remarked that the light of the mirror universe was different to that of the prime universe and that "the cosmos [had] lost its brilliance", which can be interpreted not only symbolically, referring to the dystopian nature of the mirror universe, but also literally, referring to a physical difference that would account for Terrans being more photosensitive.
In 3189, a hologram at Federation Headquarters claimed to Georgiou that in the previous century Starfleet had discovered a chimeric strain on the subatomic level in the Terran stem cell, implying that this could explain the Terran inclination towards malevolence. Georgiou dismissed this as psychological manipulation. (DIS: "Die Trying")
Society and culture[]
Millennia before the mid-23rd century, Terrans valued cooperation, freedom, and equality, but at some point decided these were dangerous fantasies that inevitably inspired rebellion, and abandoned them in favor of the fascism that eventually defined the species. (DIS: "Vaulting Ambition")
Georgiou described the Terrans as "untroubled by pesky motivations", other than revenge. Kovich observed that they created the Terran Empire for no particular reason beyond simply feeling like it. (DIS: "Die Trying")
Terran classical literature was similar in some respects to their prime universe counterparts, but differed in characterization. An exception was William Shakespeare, whose works were similar in both universes. (ENT: "In a Mirror, Darkly, Part II")