King Daniel Into Darkness wrote:
"alternate reality" is used in "In a Mirror, Darkly" by T'Pol to refer to the prime universe, a concurrently existing timeline to the mirror one.
The Mirror Universe from TOS: "Mirror, Mirror", from several DS9 episodes and from ENT: "In a Mirror, Darkly" is the first proof of the existence of a parallel universe. On the other hand, we never get to see the point of divergence. The Mirror Universe may not have been created by a time travel. For all we can imagine, it may have been severed from our own universe in its early days. For some reason, it maintains close ties to our universe, which we would not normally expect because of the second law of thermodynamics that requires the separate universes to diverge increasingly over time. The Mirror Universe is a one-off phenomenon. The somewhat parallel development to the main timeline is not only dubious in real science. It does not comply either with what happens in most time travel incidents, where the changes accumulate (the "butterfly effect"), rather than lead to the birth of visually (and genetically?) identical characters in both universes. Hence, the Mirror Universe can't really serve as a prototype for what happens in "Star Trek (2009)
But they use the same term ("Alternate reality") to describe the new movie universe (created by time travel) as they did to describe the mirror universe (more likely a spontaneous divegence as described by Data in the clip from "Parallels" I posted above ("For any event there are an infinite number of probable outcomes... anything that can happen, does happen, in alternate quantum realities"). Since the mirror universe doesn't overwrite the prime one, and Mirror-T'Pol described the Prime universe as "an alternate reality", it stands to reason that the term "alternate reality" descibes the non-overwriting parallel kind.
Remember it's fiction. They can change the rules if they want. I enjoy the novels that continue the prime universe and according to the writers of the movie and the novel DTI: Watching the Clock
, they co-exist in the Trek multiverse. It's good enough for me.