Multiverse

The multiverse, briefly referred to as the maniverse by Ralph Dibny, is a concept that refers to the existence of infinite universes that comprise everything that is. Each universe within the multiverse is referred to as a different "Earth," and each vibrates at a different frequency so that they cannot normally interact with or be seen by each other. If one is able to travel fast enough, it is possible to breach the laws of physics and travel between these Earths,\ although breaches can also be created by Vibers or by technological means. There were originally 52 known Earths in the multiverse, with an additional discovered one lacking a formal designation due to its horrific state, becoming known as "Earth-X". However, the wider scope of the multiverse includes at least 221 alternate universes, which may or may not be a part of the "local" 52-multiverse. Theoretically, for every decision a person makes, there's an alternate universe where they don't act on that decision in the same way.

History
"This is our earth. Let's call it Earth-1. And this second Earth, let's call it Earth-2. This is where Jay claims to be from. And these other Earths, 3, 4, 5, to infinity, all of them, are nearly identical to ours. And they all exist at the same time, giving us endless alternatives to what we have here. For example, the Joe West on Earth Two, your doppelgänger, he might not be a detective. He could be a Nobel Prize-winning physicist. And you, the Barry Allen here is the Flash, but over there, he could be an electrician."

- Martin Stein explaining the multiverse

In early 2388, Barry Allen and the rest of Team Flash accidentally opened a singularity above Central City; though they succeeded in closing it, a series of 52 breaches between Earths-1 and 2 were opened throughout the city as a result of their actions.

Between mid to late 2388 and early 2389, an Earth-2 speedster known as Zoom began exploring the multiverse before using the breaches to launch an assault on Earth-1's Central City in an attempt to steal The Flash's speed. Zoom sent numerous meta-humans through the breaches, each tasked with killing The Flash, but none succeeded. Team Flash, in an effort to stop Zoom's onslaught, systematically sealed each of the breaches, leaving only one open on a lower level inside S.T.A.R. Labs. Barry, Cisco Ramon, and Harrison Wells used the breach to travel to Earth-2 in order to rescue Jesse Wells, sealing the breach upon their return.

Attempting to increase his speed, Barry used a tachyon device, which in turn caused him to run so fast that he accidentally created a breach between Earth-1 and Earth-38.

Team Flash ultimately imprisoned Barry to stop him from racing Zoom and attempt to stop Zoom themselves. Though they managed to banish Zoom back to Earth-2, Joe West of Earth-1 was accidentally sent through the breach and subsequently held hostage to force Barry to agree to the race regardless.

Earth-2, along with the rest of the multiverse, was saved when Barry creates a time remnant, who sacrifices himself to thwart Zoom's plan, before Time Wraiths capture Zoom and carry him into the Speed Force for his crimes. Harry and Jesse subsequently return to their Earth, along with the real Jay Garrick, to help the latter return to Earth-3.

Later on, following the creation of Flashpoint by Barry and its undoing by the Reverse-Flash, Harry and Jesse returned to Earth-1 upon which the former immediately deduced that Barry had altered the timeline. When asked how he came to such a deduction, Harry revealed that S.T.A.R. Labs now had a speed lab that everyone remembered existing save for Harry, Barry, and Jesse, implying that Earth-2 wasn't affected by any of the timeline changes on Earth-1.

On the day Iris West was to die at the hands of Savitar, she and Joe traveled to Earth-2 and went into hiding to protect themselves. Despite their best efforts, however, Iris's location was later inadvertently revealed to Savitar by H.R. Wells, with the speedster subsequently traveling to Earth-2 himself and forcibly taking Iris back to Earth-1 to meet her death. Sometime prior to late 2391, Mar Novu began testing various Earths across the multiverse, stating that he was testing them for an impending crisis. He tested an unknown number of Earths, but after testing and decimating Earth-90, Earth-90's Barry Allen, a speedster known as The Flash, managed flee to Earth-1 to warn its heroes that their universe would be next to be tested.

In 2392, Mar Novu sent Oliver Queen to Earth-2 to recover a batch of dwarf star for the upcoming crisis. With Diggle, Typhuss and the assistance of Laurel Lance and Adrian Chase, he was successful in doing so and prepared to leave Earth-2, but later witnessed antimatter slowly consume the universe. Oliver escaped with Laurel, Typhuss and Diggle through a breach with the use of a interdimensional extrapolator.

Known parallel universes

 * Earth-1
 * Earth-2 (destroyed by antimatter)
 * Earth-3
 * Earth-4
 * Earth-5
 * Earth-6
 * Earth-7
 * Earth-8
 * Earth-9
 * Earth-10
 * Earth-11
 * Earth-12
 * Earth-13
 * Earth-14
 * Earth-15 (dead Earth)
 * Earth-16
 * Earth-17
 * Earth-18
 * Earth-19
 * Earth-20
 * Earth-21
 * Earth-22
 * Earth-23
 * Earth 23
 * Earth-24
 * Earth-25
 * Earth-26
 * Earth-27
 * Earth-28
 * Earth-29
 * Earth-30
 * Earth-31
 * Earth-32
 * Earth-33
 * Earth-34
 * Earth-35
 * Earth-37
 * Earth-38
 * Earth 40
 * Earth 41
 * Earth-47
 * Earth-48
 * Earth Fifty
 * Earth-51
 * Earth Fifty One
 * Earth Fifty Two
 * Earth-52
 * Earth 53
 * Earth 54
 * Earth 56
 * Earth 66
 * Earth 67
 * Earth 70
 * Earth 71
 * Earth-75
 * Earth-87
 * Earth 87
 * Earth-90 (destroyed by the Monitor's test)
 * Earth 92
 * Earth 94
 * Earth 95
 * Earth 96
 * Earth 97
 * Earth 99
 * Earth-221
 * Earth-260
 * Earth-494
 * Earth-827
 * Earth-898
 * Earth-X
 * An unidentified Earth (home to Hells Wells)
 * An unidentified Earth (home to the Francophone Harrison Wells)
 * An unidentified Earth (a world which nearly destroyed Earth-19)
 * An unidentified Earth (a volcanic-like Earth)
 * An unidentified Earth (an Earth where vampires are known to exist)
 * 2 undesignated Earths

Known extra-dimensional planes

 * Fifth Dimension
 * Heaven
 * Hell
 * Purgatory
 * Speed Force
 * Negative Speed Force
 * Jacob Shaw's alternate reality
 * Juru
 * World between worlds
 * Mallus's realm
 * The Monitor's realm

Pocket dimensions

 * Unnamed pocket dimension

Earth-1

 * Original timeline
 * Pre-flashpoint timeline
 * Flashpoint timeline
 * Original post-flashpoint timeline
 * Current post-flashpoint timeline
 * Legion of Doom's alternate reality
 * Elseworlds (x2)

Earth-90

 * Elseworld

Known constants in the multiverse

 * Earth
 * Humans
 * Harrison Wells
 * Henry Allen/Jay Garrick
 * Oliver Queen
 * S.T.A.R Labs
 * Big Belly Burger
 * Meta-humans
 * Speedsters
 * Vibers
 * Super-intelligent gorillas
 * Central City
 * Star City/Starling City
 * The Godfather
 * Coffee
 * Thanksgiving
 * Christmas
 * Albert Einstein
 * Beyoncé Knowles
 * Mariah Carey
 * Al Capone
 * Super Mario
 * Peanuts
 * Harry Potter
 * Star Wars
 * Ghostbusters
 * Hamilton (musical)
 * Krypton
 * Dominators
 * World War II
 * Nazis
 * Hitler
 * Atomic bombs

The multiverse in Star Trek
The multiverse is the sum total of all existence, the compiled mass of every universe and presumably other extra-dimensional realms such as fluidic space.

Very few species have the capabilities to explore the entire multiverse and the vast majority stay well within the confines of their own universes. One species that can and readily do traverse the mulitverse are the Q. (TNG novels: Q-Zone, Q-Space, Q-Strike)

History
In the War of the Prophets alternate timeline, Miles O'Brien posited that there had to be multiverses where the universe did not end in the year 2400. (DS9 - Millennium novel: Inferno)

In 2380, all Kathryn Janeways in the multiverse died through various means. Her death became a fixed point in time the Q dared not change. However, Q saved Janeway from oblivion in the primary universe and aided in her return to the living in mid-2381. (VOY novel: The Eternal Tide)

List of universes

 * primary universe
 * at least 285.000 quantum realities
 * various timelines (ST - Myriad Universes novels: Infinity's Prism, Echoes and Refractions, Shattered Light)
 * Yesterday's Enterprise timeline (, STO mission: "Temporal Ambassador")
 * mirror universe
 * various permutations (, TNG novel: Q & A)
 * alternate realities
 * Kelvin timeline (TOS movie & novelization: Star Trek)
 * mirror Kelvin timeline (TOS comics: "Mirrored, Part 1", "Mirrored, Part 2")
 * parallel alternate reality (TOS - Parallel Lives comic: "Part 1")
 * antimatter universe
 * fluidic space
 * home of the nucleogenic lifeforms (VOY episode & novelization: Equinox)
 * Omega Continuum (VOY novel: The Eternal Tide)
 * subspace (,, STO mission: "Devil's Choice")
 * trans-dimensional realm
 * Q Continuum

Season 2

 * "Flash of Two Worlds" (mentioned)
 * "Welcome to Earth-2"
 * "Escape from Earth-2"
 * "The Race of His Life" (mentioned)

Season 3

 * "The New Rogues" (mentioned)
 * "The Present" (mentioned)
 * "Dead or Alive" (mentioned)
 * "Attack on Gorilla City" (mentioned)
 * "Attack on Central City" (mentioned)
 * "Into the Speed Force" (mentioned)
 * "Duet" (mentioned)
 * "Abra Kadabra" (mentioned)
 * "Cause and Effect" (mentioned)
 * "Infantino Street" (mentioned)
 * "Finish Line" (mentioned)

Season 4

 * "Elongated Journey Into Night" (mentioned)
 * "When Harry Met Harry..." (mentioned)
 * "Therefore I Am" (mentioned)

Season 1

 * "Worlds Finest" (mentioned)

Season 5

 * Missing (mentioned)

Trivia

 * The fact that the number of breaches that opened in Central City is 52 is a reference to both the multiverse crisis storyline known as 52 and The New 52 imprint in the DC Comics.
 * During an interview, the producers of the NBC television show Powerless, jokingly referred to the Multiverse by saying that their show takes place on "Earth-P" as it was a DC Comics comedy. This is of course, unofficial and they actually revealed, at the time of the interview, that there was no plans to crossover with any other DC property, the show has since been cancelled.
 * On numerous occasions, it has been stated that there are an infinite number of Earths in the Multiverse. On the other hand, Kara Danvers claimed that there were 52 of them. However, Harrison Wells quickly corrected her, describing Earth-X as a 53rd Earth, indicating that, aside for Earth-X, there are 52 known Earths in the multiverse.