I reject the premise of the question, because the difference between them is one of nationality, not species. They are Federation humans and Imperial humans. The thing is, the term "Terran Empire" was never canonically used until Enterprise, and I consider that a mistake. When DS9 had the Alliance members refer to humans as "Terrans," it was evidently intended by the writers as a slur, a term that aliens used derogatorily. That's often how "Terran" has been used in science fiction, as a term aliens use for humans. And in this case it was inaccurate, since not all humans are from Earth, so that compounds its nature as a term used by outsiders who don't care about the distinction. So it was a retcon, and IMO a misinterpretation, for ENT to start using it as the Imperial humans' term for themselves. And DSC has just compounded that error by treating it as a species name. It would've been preferable if they'd stressed the "Empire" part instead of the "Terran" part, because it's the "Empire" part that's the meaningful difference from Federation humans. So I'd prefer it if they'd been called Imperials instead of "Terrans," because Federation Earthlings are Terrans too.
I think the name "Terran Empire" was chosen, in-universe, because the Empire just thought it sounded good. (They have to call themselves something, anyway. ) And that, in turn, was the very reason the Alliance turned it into a slur - throwing it back in the humans' faces, as it were. Gloating over the fact that the former citizens of the Empire are now slaves. And I greatly prefer "Terran Empire" over what the novel Dark Mirror called it - "United Empire of Planets". I mean, DUH, of course it's united, it's a fucking empire! That said, I agree that it was a mistake for the DSC writers to use "Terran" as a species name. I'm totally in favor of retconning that out of existence. Which it seems they might have already done - in "Die Trying" when the holo-doctors are talking to Georgiou, she immediately dismisses any suggestion that "Terrans" are any different, biologically, from humans. "You cannot rattle me by introducing a fabricated biological component to my nastiness," and all that.
Of course the difference is one of nationality, not species. That is not in question. The issue is, the writers of DIS needed a short name to use to distinguish between Prime Universe Humans and Mirror Universe Humans, one that was a plausible informal idiom in keeping with DIS's rejection of Berman-era Trek's use of highly stylized prescriptive English. So I don't think you can reject the premise -- the writers clearly needed a term to distinguish between the two different Human cultures that would be in keeping with the linguistic conventions they had previously established for their characters. I think we just have a fundamental difference of opinion here and I think this is a subjective aesthetic thing. I didn't get the impression "Terran" was meant as a slur in the DS9 MU episodes, and I think it's quite believable that the MU Humans, in adopting some of the iconography and terminology of the Roman Empire (presumably in order to lend an air of legitimacy and ancient authority to their system of government), might adopt the word "Terran" to refer to themselves and their fascistic culture. (Indeed, it strikes me as similar to how the Nazis adopted the term "Aryan" for themselves, even though early 20th Century Germans had next to nothing to do with ancient Indo-Iranians.) ETA: I mean, "United" may not be a good adjective to use in the name, but, to be fair, I am intrigued by the implications of the name. "United Empire of Planets" implies a different version of authoritarianism. Whereas "Terran Empire" makes it clear that it's an imperial structure under the dominion of one particular culture, "United Empire of Planets" implies it's an imperial structure whose ruling class is not from any one culture but instead from a fusion of cultures -- a diverse ruling class, in other words, that recognizes the artificiality of its political structure rather than a ruling class built on bigotry. Not less evil, but a different kind of evil.
^ I dunno, I guess I'm just a fan of keeping things short and to the point. "Terran Empire" just kind of rolls off the tongue. Efficient, quick and brutal - like the Empire prefers. It's like the "other" Alliance, the one from Firefly. "Union of Allied Planets"? Uh, I think we got that, how can it be a union if the planets aren't allied? I actually do get the basic thrust of your gist. I admit I didn't even consider that angle.
Why wouldn't it be? It was used by the aliens who had conquered and enslaved humanity. When have enslavers ever referred to their slaves' race with respect?
Yeah, it's a redundant name, but it might be fun to try to rationalize it. Maybe it's called that to represent the idea that these allied worlds, formerly separate, had formed a single sovereign state? Still not as redundant as them calling it "the Klingon Imperial Empire" in TNG "Sins of the Father" though. I mean, sure, but did any of the Mirror Universe Humans in DS9 ever react with offense towards the term? I don't recall it if they did.
But did they ever indicate to the Prime Universe characters they found the term offensive? Did they ever show even a non-verbal indication of offense? Did they ever talk amongst themselves about not liking the term? I honestly just don't remember any sequence from DS9 to indicate that they found the term offensive. Side-note: I just noticed that @Christopher edited one of his posts above to indicate that he would have preferred for the DIS writers to use the term "Imperials" for Mirror Universe Humans from the Terran Empire. Thank you for the clarification.
Uhhhhh... They had grown up as slaves their entire lives. "React with offense?" Do slaves have the privilege to express that to their masters?
As I said above: But did they ever indicate to the Prime Universe characters they found the term offensive? Did they ever show even a non-verbal indication of offense? Did they ever talk amongst themselves about not liking the term? * * * Is there anything whatsoever to indicate that "Terran" is a slur? If that's your subjective interpretation, that's legitimate, but that's not per se the same thing as the idea actually being present in the text.
You're really not giving much thought to what it means to be a slave. Everything about it would offend them! They shouldn't have to say it!
You are making a Watsonian argument on the basis of an a priori assumption that I am not convinced is present in the text. I am arguing from a Doyalist perspective that that a priori assumption may not be in the text. Obviously, if the term "Terran" were a slur in the Mirror Universe, it would be deeply offensive to those characters. But I am not convinced at all that it is a slur. I can recall no scene in which the MU Humans give any indication they find the term offensive to a Prime Universe character, nor any scene amongst themselves where they do something to indicate offense at the term (for example, complaining about Cardassian or Klingon oppressors using the term). I can't even remember a scene where they react nonverbally to the term. If I'm wrong and such scenes were there, then great! I'd be with you. But I'm not persuaded the idea is reflected in any of the writing or acting choices or that the idea was the intention of any of the writers or actors. I'm not persuaded the idea that "Terran" is a slur in the 24th Century Mirror Universe was on any of the creators' minds. If you can cite a particular scene to indicate that the idea is present in the text -- again, for instance, a nonverbal negative reaction to the word in an actor's performance, or a scene where the MU Humans indicate verbally to a Prime Universe character that it's offensive -- then I'll happily concede the point. But unless you can, I think at this point we're just going in circles and I'll bow out.
No, I don't recall anything like that ever happening. And I realize there's no proof that the Alliance intended "Terran" as a slur, that's just the vibe that I've been getting from how they speak. Especially the Intendant - she oozes scorn and menace every time she talks to a human.
Discovery: [changes and retcons pretty much everything about Trek] Christopher: How dare they change [very specific thing]!
It's not about whether a thing was changed. It's about whether a change makes sense or not. Quality is not about category, it's about individual cases. You should remember by now how much I hate blanket generalizations.
When Cardassians call the people whose planet they occupied Bajorans, as Dukat and others have done onscreen multiple times, is Bajoran considered a pejorative term?
But that's actually used as a species name by the Bajorans themselves. "Terran" doesn't mean "human," it means "inhabitant of Earth." Since not all humans are from Earth (a fact that Trek admittedly tends to forget), using the terms as synonyms would be inaccurate and culturally insensitive. It would be like referring to all Asian people as "Chinese" even if they're from Japan or Laos or Tibet. Which would definitely be an ethnic slur in that context. Of course, you can spin all sorts of rationalizations to say it could be their name for themselves, but that's not what I'm saying. I'm saying what my impression of the writers' original intent was in "Crossover," if you strip away all the retroactive assumptions based on how it was used in later Trek. And at the time of "Crossover," it was a long-standing practice in science fiction to use "Terran" as an alien term of address for humans. For instance, in TOS, the only times the word was ever used was by Vulcan children in "Yesteryear," the Romulan Commander in "The Enterprise Incident," and Vixis in ST V. It was infrequently used by humans in TNG, but historically, SF has commonly treated it as an alien usage, meant to sound exotic, distancing, and overly formal or technical, like someone referring to a cat as a felinoid. And its use in "Crossover" felt like another example of that same practice, with the added dynamic of conquest and enslavement making it derogatory. Thus, the decision of later writers (mainly Judith & Garfield Reeves-Stevens, who coined "Terran Empire" in the Shatnerverse novels and then joined the ENT writing staff in season 4, when it was first used in canon) to make it the Imperial humans' term for themselves feels revisionist to me.
Actually, "Terran Empire" goes at least as far back as Ruth Berman's fanfic short story, "Visit to a Weird Planet Revisited," published (and thereby promoted to pro-fic) in Star Trek: The New Voyages, almost two decades before the first Shatnerverse novel. It can also be found online here. Also, there are novels and short fiction that establish that there was at one time a Bajoran ethnic group called the Bajora, one of several ethnic groups. Of course, none of that has ever been pulled into canon (at least so far). It kind of reminds me of a scene from the Infocom game, Beyond Zork, of an ancient conflict between the residents of the cities of Pheebor and Borphee, over what to call the river formed by the joining of the rivers Phee and Bor. (Pheebor lost.)