A multiverse is, as its name implies, a collection of multiple universes (of the second type). A multiverse can have anywhere from 2 to an infinite amount of universes. Several modern scientific theories, such as M-Theory, postulate that our real universe is in fact part of a multiverse.
A larger set above a multiverse, containing multiple multiverses, is often called a megaverse. Examples include the various megaverses from Marvel, Image and DC (formed of their various publishing and film ventures), and the megaverse formed by the Downstreamers in the Manifold series.
The term "omniverse" was coined by Marvel Comics editor Mark Gruenwald (at the time working for a fanzine) and at the time of its creation the term was taken to mean everything in all of fiction as well as real life. In popular usage the term has (d)evolved, however, so that within an in-universe context it is often used to mean a set of every dimension, realm, universe, multiverse, megaverse, plane of existence, timeline, continuity, actuality, and reality within a work, series franchise, or fictional setting. Marvel Comics has adopted Gruenwald's scale, and explicitly acknowledges an omniverse, which has been referenced and supported multiple times. However Marvel Comics is also known for its changing of the term. At one point it was considered that the Marvel Omniverse was equal to all realities, all universes, planes, dimensions. It was considered to be everything, literally everything without exception (see the discussion on it at KMC ). In the OBD however, this is usually disregarded, and is kept down to its in-universe parameters.
Feats that involve creating/destroying/affecting multiverses and up are considered, as a general rule, to be greater than universal feats. All of the strongest fictions have at least a multiverse (there might possibly be exceptions, but I haven't seen one yet).
Fictions that have Multiverses:
- 8-bit Theatre
- Ah! My Goddess
- Akira (implied)
- Bokurano
- Bionicle
- Buffy the Vampire Slayer
- Chronicles of Narnia, The
- Crossgen comics
- Cthulhu Mythos
- Culture, The
- Dark Horse Comics
- Dark Tower, The
- DCAU
- DC Comics
- Demonbane
- Digimon
- Disney
- Doctor Who
- Dual! Parallel Trouble Adventure
- El-Hazard
- Family Guy
- Final Fantasy
- Futurama
- GetBackers
- Heroic Age
- Hyperion Cantos (implied)
- Illium/Olympos
- Image Comics
- Jojo's Bizarre Adventure
- Kamen Rider
- Katekyo Hitman Reborn!
- Kids Next Door
- Lost Universe
- Magical Girl Lyrical Nanoha
- Magic: The Gathering
- Manifold series
- Mario Bros
- Marvel Animated
- Marvel Comics
- Mortal Kombat
- MS Paint Adventures
- Nasuverse
- Neon Genesis Evangelion (implied)
- Nipponverse
- Noein
- Paper Mario Series
- Pokemon
- Project A-ko
- RahXephon
- Rozen Maiden
- Saint Seiya
- Saiyuki
- Shin Megami Tensei
- Slayers
- Stargate
- Star Trek
- Sonic the Hedgehog
- Tenchi Muyo!
- That Guy with the Glasses
- The One
- Time Odyssey, A
- Transformers
- TTGL (implied)
- Touhou
- Umineko no naku koro ni
- Valiant Comics
- Vertigo Comics
- Wildstorm Comics
- Xeelee Sequence
- Xenogears
- Xenosaga