Multiverse

A multiverse was a grouping of parallel universes. "The Multiverse" was sometimes used to refer to the totality of all possible universes, but according to other accounts, this was more properly referred to as the Omniverse while the term "multiverse" referred to comparatively smaller — though still infinite or near-infinite — structures within it.

Structure
The empty space between dimensions was known as the Void. (TV: Army of Ghosts, etc.)

According to some accounts, "multiverses" were smaller groupings within the Omniverse which comprised every extant multiverse. Esterath told the Eighth Doctor that in all his past travels across dimensional planes he had never left his own multiversal realm. Having been allowed to experience the lives as his alternate selves, the Doctor realised that together they comprised one being. (COMIC: The Glorious Dead) The Tenth Doctor claimed that Anubis's ascension would collapse the Circle and destroy not just the Multiverse, but multiple other Multiverses as well. (COMIC: Sins of the Father)

By one such account, the multiverse of the Doctor's reality existed in a state of quantum superposition which could be violently collapsed into a single crystalised web of time. The Council of Eight's attempt to effect such a thing once cause a multiverse collapse which temporarily destroyed all realities save their own. (PROSE: Sometime Never..., Time Zero)

Other accounts used "the Multiverse" to refer to all possible universes. (AUDIO: Master of Worlds, Palindrome)

Nature
The Toymaker claimed that he existed in the universe because all beings in the multiverse dreamed, and he shaped those dreams lest they become stale. (PROSE: Divided Loyalties)

Darlington, a town in North-Eastern England, was a gateway to the multiverse, and the Darlington branch of MIAOW operated there to monitor the gateway. (PROSE: Enter Wildthyme)

Travel through the multiverse
Iris Wildthyme's Celestial Omnibus could travel through the multiverse. (AUDIO: Wildthyme at Large, et al)

Origins
When the multiverse was young, the Sentients, the Modulars, the Clockworks, the Recursives, the Binaries and many more species were all allies. (PROSE: Elementary, My Dear Sheila) The people of the Clockworks were responsible for maintaining the basic structure of the multiverse. (PROSE: The Blue Angel)

Under the Time Lords
During the Eighth Doctor's lifetime, the last of the Daleks became concerned about the idea that aberrant Daleks from other universes could cross over into their reality and supplant them. The Supreme Dalek initiated a complex plan to suborn Crivello's Cauldron and turn it into a gateway into the rest of creation, allowing a preemptive strike on the alternative Daleks. However, opening the wormhole would in reality have caused Abaddon, the collapse of all universes into a single hellish nothingness. The Doctor was able to put an end to the Daleks' plans, destroying them and sealing the rift. (COMIC: Fire and Brimstone)

War in Heaven
Even though The Book of the War believed that travel between different universes was likely impossible and thus claimed the War in Heaven would be confined to the universe of the Spiral Politic, (PROSE: The Book of the War) the War touched other realities. (PROSE: The Brakespeare Voyage, Warlords of Utopia)

Last Great Time War
During the Last Great Time War, the Valeyard managed to wipe the Daleks from N-Space, the Dalek Time Strategist escaped into the Multiverse. (AUDIO: The War Valeyard) Using a transdimensional portal built by an alternate Davros, the Time Strategist managed to use a merging of echoes of infinite realities to resurrect the Dalek Empire (AUDIO: Palindrome) and return them to N-Space. Having futilely scoured the many alternate realities for the Dalek Prime, the Strategist instead resurrected the original Dalek Emperor via the energies of the multiverse. The connection to the multiverse initially allowed the Daleks to call on their counterparts as infinite reinforcements before the Eighth Doctor destroyed the equipment sustaining this, returning all the alternate Daleks to their realities. (AUDIO: Restoration of the Daleks)

During the Time War, encountered the Cybermen of the Cyber-Mainframe, who originated from a parallel universe and sought to upgrade the entire multiverse; these Cybermen used the term "the multiverse" to refer to all realities in existence. (AUDIO: Master of Worlds)

On the final day of the Time War, Rassilon planned to destroy the whole of creation with the Ultimate Sanction. To escape the destruction of reality, the Time Lords would shed their corporeal bodies and become creatures of consciousness alone, ones that would escape the effects of time, cause and effect. They were defeated by the Tenth Doctor before they could do so. (TV: The End of Time)

Post-War in Heaven universe
In the post-War universe, the Council of Eight attempted to effect a new anchoring of a Web of Time, but overshot and caused a collapse that left their specific version of history as the only extant reality in the Doctor's multiverse. (PROSE: Sometime Never...)

Post-Time War universe
The effects of Davros' reality bomb travelled through the time rift in the Medusa Cascade to affect the multiverse. (TV: Journey's End) The bomb weakened the barriers between universes allowing Rose Tyler to begin travelling across the multiverse to try and return to N-Space and warn the Doctor of the bomb. (AUDIO: The Endless Night) Her travels ultimately resulted in the prevention of the bomb's detonation and the walls of the universe eventually started closing themselves, a process that the DoctorDonna called a dimensional retroclosure. (TV: Journey's End)

Anubis attempted to use the Circle of Transcendence to ascend to a higher dimension to join the rest of the Osirans, but the technology maintaining the Circle had been degraded too much, and the Doctor claimed that Anubis' ascension would collapse the Circle and destroy not just the Multiverse, but multiple other Multiverses as well. (COMIC: Sins of the Father)

A Cyber-Leader at Stonehenge warned that with total event collapse, "all universes will be deleted", indicating that the phenomenon would erase the whole multiverse from existence. (TV: The Pandorica Opens)

The Fractures were described as anti-bodies for the multiverse, finding any threats and destroying them. This even included crossing through the Void. The Fractures attempted to hunt down Paul Foster after he used Cybermen technology to cross universes, but were defeated by the Twelfth Doctor and sent back into Void. (COMIC: The Fractures)

Behind the scenes
The video game LEGO Dimensions and the feature film The LEGO Batman Movie, which are not considered valid sources on this Wiki, confirmed the notion of the Doctor's world (and, presumably, the various parallel worlds the Doctor has visited in the past) belonging to the wider multiverse seen in other LEGO media, which includes versions of the realities of franchises such as The Lord of the Rings, DC Comics, Back to the Future, Mission: Impossible, Ghostbusters, The Simpsons, Scooby Doo, Sonic the Hedgehog, E.T. the Extra-Terrestrial, Harry Potter, The Wizard of Oz, and Jurassic Park.

The multiverse concept has also been used over the years by fans to link disparate aspects of the expanded franchise, such as novels, audios and the Peter Cushing films, to the "canonical" universe of the televised franchise. Big Finish also published a line of audio dramas explicitly set within alternate realities: see Unbound Universe.