A multiverse was a grouping of parallel universes.
The omniverse 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) It was also known as reality the empty spaces between the universes was known as the Void with the parallel universes arranged together with the Void acting as the space inbetween. (TV: Army of Ghosts)
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)
Iris Wildthyme's Celestial Omnibus could travel through the multiverse (AUDIO: Wildthyme at Large, et al) and the Axis was capable of allowing an individual to travel through the multiverse via portals that led to different timelines. (AUDIO: Reborn)
By one account, the multiverse existed in a state of quantum superposition which could be violently collapsed into a single crystalized web of time. The Council of Eight caused a multiverse collapse which temporarily destroyed all realities save their own. (PROSE: Sometime Never..., Time Zero)
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)
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. (AUDIO: Restoration of the Daleks)
During the Time War, the War Master 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)
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)
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)
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.