Multiverse (Marvel Comics)
Within Marvel Comics, most tales take place within the fictional Marvel Universe, which in turn is part of a larger multiverse. Starting with issues of "Captain Britain", the main continuity in which most Marvel storylines take place was designated Earth-616, and the Multiverse was established as being protected by Merlyn. Each universe has a Captain Britain designated to protect its version of the British Isles. These protectors are collectively known as the Captain Britain Corps. This numerical notation was continued in the series "Excalibur" and other titles. Each universe of the Multiverse in Marvel also appears to  be defended by a Sorcerer Supreme at nearly all times, appointed by the mystic trinity of Vishanti to defend the world against threats primarily magical in nature from within and beyond and bearing the Eye of Agamotto. Later on, many writers would utilize and reshape the Multiverse in titles such as "Exiles", "X-Men", and "Ultimate Fantastic Four". New universes would also spin out of storylines involving time-traveling characters such as Rachel Summers, Cable, and Bishop, as their actions rendered their home times alternate timelines. The Multiverse also plays a role in the Marvel Cinematic Universe (MCU), also known  as Earth-199999 in the comics Multiverse. Its concept was first introduced in "Doctor Strange" (2016) before being explored more deeply in of the MCU. Additionally, Sony's Spider-Man Universe (SSU) ties into the MCU multiverse. Concept. The Multiverse is the collection of alternate universes that share a universal hierarchy. A large variety of these universes were originated from another due to a major decision on the part of a character. Some can seem to be taking place in the past or future due to differences in how time passes in each universe. Often, new universes are born due to  time traveling; another name for these new universes is an "alternate timeline". Earth-616 is the established main universe where the majority of Marvel books take place. The Marvel multiverse is protected from imbalance by the Living Tribunal, a vastly powerful humanoid cosmic entity, who is one for the entire multiverse. It may act to prevent one universe from amassing more power than any of the others or from upsetting the cosmic balance in some way. It is only overseen by the One-Above-All, an omnipotent entity said to have created the entire Marvel Multiverse. The origin mythology of the  Multiverse in Marvel Comics is that the Celestials, members of the first lifeforms created in the original universe, contributed to the creation of the Multiverse, including the ongoing evolution of life therein. Nature of the Multiverse. According to Forge, mutants living on these alternate Earths have lost their powers due to M-Day, as stated in ""; however, this mass depowering has not been seen in any of Marvel's current alternate reality publications such as "Exiles", the "Ultimate" titles, "Amazing Spider-Girl", the "Marvel Adventures" titles or "GeNext", though it is possible that the issue of time may be related  to their exclusion. This was apparently retconned during the "" storyline, where Forge stated that all mutants in possible future timelines were depowered, not in parallel universes. This, in addition to A.R.M.O.R.'s observation that Lyra arrived from an alternate reality indicates that the topology of the Marvel Multiverse is based on new realities branching off from key nodes of a timeline instead of strictly parallel dimensions. Other realities. Not every alternate reality is an entire independent universe, but instead maintain a parasitic relationship to a parent reality. Others can exist outside the multiversal structure altogether. Definitions. The classification system for  alternate realities was devised, in part, by Mark Gruenwald. Continuity. A Universe/continuity is a single reality, such as Earth-616, the mainstream Marvel Universe/Continuity. In Marvel Comics, the concept of a continuity is not the same as "dimension". For example, demons like Mephisto and gods like Odin hail from separate dimensions, but they all nevertheless belong to Universe-616. A continuity should also not be confused with an imprint; for example, while the titles of some imprints, such as Ultimate Marvel, take place in a different continuity, some or all publications in other imprints, such as Epic Comics, MAX, and  Marvel UK, take place within the Earth-616 continuity. Note that whether any given specific use of the term "Marvel Universe" refers to the Marvel "Multiverse" (in general) or to the Earth-616 continuity (in particular) can only be determined by the context of its use. Multiverse. A multiverse is the collection of alternate universes, with a similar nature and a universal hierarchy. The Marvel Multiverse contains the universe that holds Earth-616, most of the "What If"? universes, as well as the vast number of the alternate Marvel Universe Earths. Megaverse. A Megaverse is a collection of alternate multiverses, which do not  necessarily need to have similar natures and universal hierarchies. The term was posited in the 21st century edition of the "Official Handbook of the Marvel Universe". Omniverse. According to the "Official Handbook of the Marvel Universe: Alternate Universes", and building on Mark Gruenwald's original definition of the term, the Omniverse consists of all of fiction and reality combined, including all the works that are outside of Marvel's copyright restrictions. As such, there can logically only be one Omniverse, as anything and everything that currently exists, existed in the past, can potentially exist at any time or may exist  in the future is a part of it. Known alternate universes. As stated above, nearly every imprint, timeline and appearances in other media have its own separate universe. Most of these have been cataloged by Marvel Comics in many publications, being most notable the "Official Handbook of the Marvel Universe: Alternate Universes". The numerical designations for these are rarely revealed outside of reference works such as the "Official Handbook of the Marvel Universe: Alternate Universes 2005". A.R.M.O.R. and Project Pegasus however seem to possess vast knowledge of other Marvel realities, utilizing the same designations; whether this is simply narrative convenience  on behalf of Marvel's authors or an unusual decision by these agencies to utilize an effectively alien catalog method is as yet unstated. The numeric designations of these alternate universes have been confirmed by Marvel Comics throughout the years and compiled in 2005's "Official Handbook of the Marvel Universe: Alternate Universes", and in Marvel publications since the release of the "Handbook". The prevalent method of labeling an unnamed universe is to derive numbers in some way from the publication date of the relevant issue featuring its first appearance. This is, in turn, based on the mistaken belief that  Earth-616 derived its number from the publication date of "The Fantastic Four" #1 (Nov. 1961), although the origin of this term in particular has been under debate. One active theory is that it came from publications by Marvel UK from Saturnyne mentioning that Captain Britain was from Earth-616. Many official numbers are random or use other numbers as a base, the best example of this is Ultimate Marvel. 1610 is the swapped numbers of 616 with a 0 to differentiate it from the already existing 161. In addition, many universes have also been designated with numbers by fans with various  methods for the numbering, such as the birth date of an important Marvel staff member (artist Nelson Ribeiro for the Transformers U.S. Universe, Earth-91274) or the spelling of a name with a touch-tone phone (Animated Silver Surfer Earth, 936652, spells out Zenn-La). In 2014, during the publication of the "Spider-Verse" storyline, writer Dan Slott posted on Twitter that the numbers that appear in wiki entries and handbooks do not count, only those that are published within "actual" stories do. This was in response to the questions that the different numbers for some Earths appearing in "Spider-Verse  brought up, such as the Spider-Friends being from Earth-1983 and not the believed designation of Earth-8107. This has created some debate among readers, as some believe that the Spiders" with numbers that do not match the "original" ones are alternate versions, or if the former numbers should be completely dismissed, despite being official. In the 2015 "Secret Wars" series, a confrontation with the Beyonders over the fate of the various alternate versions of the Molecule Man results in the destruction of the Multiverse, triggering various 'incursions' as Earths crash together and destroy each other, the Beyonders' assault culminating in  Doctor Doom stealing the power of the remaining Beyonders and bringing the last of the parallel universes together into a single 'Battleworld'. Doom rules this reality for eight years until key heroes and villains from the pre-existing Multiverse are discovered and released by Doctor Strange, who had been acting as Doom's 'sheriff' until the discovery of the survivors gave him an alternative. In the heroes' final assault on Doom's fortress, the Molecule Man, who had been the source of Doom's power, transfers Doom's power to Mister Fantastic when Doom acknowledges that Reed would have done  a better job as 'God' than he did. Having restored Earth-616 as it was before the Beyonders' incursions began, Mister Fantastic departs to recreate the multiverse with the company of his restored family. List of main alternate Earths and universes. Below is a short, non-comprehensive list of the main universes in the Marvel Multiverse: