Multiverse

The multiverse is a hypothetical group of multiple universes, collectively comprising everything that exists. This concept is the basis for the Spider-Verse, or multiple Spider-based characters existing simultaneously in parallel universes.

In Marvel Media
The multiverse has been featured repeatedly within various Marvel canons, both directly and through inference, to a point that an effort to summarize the entirety of it would be absurd. In 1977 Marvel began to publish a series titled What If, which explored alternate dimensions with a single point of divergence from the main canon of Earth-616. Characters such as the Fantastic Four and Doctor Strange made a habit of exploring alternate realities long before even this. Retellings, reboots, and adaptations almost perpetually created new alternate continuities, which are almost universally treated as their own universes.

Spider-Verse
A Spider-centric focus on the multiverse arguably began with the 1994 cartoon Spider-Man: The Animated Series. The final two episodes of this series, known as the Spider-Wars duology, detailed six alternate versions of Peter Parker coming together to thwart the homicidal plans of an evil Parker.

The next major exploration of the Spider-Verse was the 2010 video game Spider-Man: Shattered Dimensions, which featured three versions of Peter Parker along with Miguel O'Hara. Withing this game's story, a scuffle between Spider-Man and Mysterio results in the shattering of a powerful artifact called the Tablet of Order and Chaos, the pieces of which are scattered through four different realities and must be retrieved. This story was penned by Dan Slott, who four years later would write the event comic simply titled Spider-Verse. More on that later.

Though comic books had acknowledged the multiverse many times before, Spider-Men, a 2012 miniseries written by Brian Michael Bendis, was significant for what it would later inspire--effectively thrusting Miles Morales into the Spider-Verse's center stage for the first time. Two years later would come the Spider-Verse series, which featured a multiversal army of spider-powered heroes locked in battle with the extradimensional, vampiric Inheritors, and which was the first Spider-Man crossover of its scale. It was followed up with a series titled Web Warriors and an eventual sequel. A year later, Marvel's major flagship dimensions collided in the 2015 Secret Wars event, eventually coalescing into a single reality with a new status quo assembled from those of its constituent worlds.

In 2018, the animated movie Into the Spider-Verse detailed the intersection of six different realities, as experiments with a supercollider in one universe accidentally summon the Spider-heroes of five others. The end of the film featured Miguel O'Hara constructing a device which allows for the safe transportation to alternate realities, which look to be featured prominently in the 2022 sequel.

Fictional Structure
Although an exact consensus on the multiverse's structure within the fiction of Marvel is in flux, with details varying wildly between creative teams and across adaptations, a kind of broad understanding has been more or less agreed upon. Existence is understood to be divided into portions called dimensions, which can be nearly infinite in space and known as universes or smaller, demonstrably finite realms. Dimensions can also be classified by their composition: "Earth-like" dimensions have similar makeup, physical properties, and laws of physics to those possessed by what we consider to be Earth, while "alien" dimensions have different physical properties entirely, and "magic" dimensions have physical properties and laws governed by forms of magic. There is an informational space between dimensions known as the Superflow, where no known laws of physics apply and where ancient machines from unknown sources drift endlessly. Almost nothing is known for sure about this place.

Dimensions can and often do form associations and loose bonds with each other; a collection of dimensions entwined like this is known as a reality, and usually (but not always) contains a single Earth-like universe. These realities are categorized via numbering; the most correct way to refer to a given reality is as "Reality-XXXX" (with the unique numeration of said reality replacing "XXXX"). Due to the audience's placement on and familiarity with Earth, references to a given reality commonly use the prefix "Earth-" instead. Reality-616, for instance, is usually called "Earth-616."