Memory Alpha
Memory Alpha
Multiple realities
(covers information from several alternate timelines)

Elsewhere in the multiverse...

The multiverse was the set of parallel universes, which included the prime universe and the Mirror universe, as well as theoretically many others. On the physics level, matter from different universes differed by having a different quantum signature. (TNG: "Parallels")

Several methods of crossing from one universe to another existed, including traveling along the mycelial network, certain transporter accidents, and interphasic rifts.

Assuming the realities seen in "Parallels" are also other universes within the multiverse, quantum fissures provide another way.

The mycelial network was believed to underpin all quantum realities, extending across the entire multiverse. This proved to be a threat when Paul Stamets of the Mirror universe built a reactor which corrupted it, which threatened to destroy the network in all realities, and with it all life. (DIS: "Despite Yourself", "What's Past Is Prologue")

"Parallels", "Despite Yourself", and to a lesser extent "Vaulting Ambition" use very similar language establishing different quantum signatures as inherent markers defining different universes. This likely suggests that the realities seen in "Parallels" are additional universes of the same nature as the Mirror universe, or put differently, that the terms "Parallel universe" and "quantum reality" are in fact synonyms. Despite Yourself provides another hint as to that in that Stamets talks about the mycelial network extending along, quantum realities after finding himself in the Mirror universe, which has generally been described as a parallel universe.
It has been suggested as being possible that the alternate reality created by Nero's incursion in 2233 also exists within this multiverse system, but evidence more strongly supports that a mere disruption of continuity in a single timeline may not create a parallel universe. (DIS: "What's Past Is Prologue"; Star Trek)

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