Parallel universe

A parallel universe, (PROSE: The Face of the Enemy) parallel world (TV: Utopia) or parallel reality (AUDIO: Faustian) was a universe which existed separately from the one usually travelled by the Doctor. The Tenth Doctor once explained that there were "billions of parallel universes, all stacked up against each other", with the Void as the space "in-between". He used the term of "parallel universe" interchangeably with that of "different dimension". (TV: Army of Ghosts)

explained that a parallel Earth occupied "the same space-time coordinates as this Earth, but in a different dimension. Sideways in time, if you like, rather than forward or back." (PROSE: The Face of the Enemy) "All possibilities" occurred across the sum of all existing parallel realities, (AUDIO: Faustian) meaning some parallel realities resembled each other with 'points of divergence', similar to an alternate timeline, (TV: Rise of the Cybermen) although those tended to overwrite the previous version of history rather than exist distinct from it. (PROSE: Imperial Moon)

The sum of all existing parallel universes was often called the Multiverse, (AUDIO: Restoration of the Daleks) although according to another account, a multiverse was a smaller grouping of parallel universes, and the true sum total of all Creation was called the Omniverse. (COMIC: The Glorious Dead)

Physics
The Third Doctor once described travel between parallel universes as "like travelling sideways" rather than on a nature course. (TV: Inferno)

As the Fifth Doctor once explained to Turlough, parallel universes were a specialised form of alternate universe — "alternative universes [that existed] in parallel with each other" — but the terms weren't complete synonyms. A parallel universe had no impact upon another parallel universe; it was "separated only by a millisecond of time and a nanometre of space, without ever having contact." Conversely, a fully alternate timeline was when, because of the inherent weakness at the point that one universe split from the other, one timeline actually destroyed the other. (PROSE: Imperial Moon)

At least some parallel universes were created as a consequence of people making different decisions — the Tenth Doctor explained to Jackie Tyler that "every single decision we make creates a parallel existence". (TV: Doomsday) The Tenth Doctor also said that there were "billions of parallel universes all stacked up against each other. The Void is the space in between, containing absolutely nothing." (TV: Army of Ghosts) However, the Eighth Doctor noted that such significant events remained indeterminate until the act of observing them forced the universe to "choose" the outcome, with the new timeline only splitting if absolutely necessary, otherwise creating "oxbow lakes of might-have-been". (PROSE: Time Zero)

Time behaved differently in some parallel universes compared to the Doctor's World. In one universe, the Inferno Project was more advanced. (TV: Inferno) Pete's World was three years ahead, (TV: Doomsday) while William Blake spent a few days in the Spiral Yssgaroth and returned to Earth seventy years later than when he left. (PROSE: The Pit, The Book of the War) It was possible to align the timelines of another dimension with Gallifrey's, which would render travel from one to the other much simpler, and indeed, essentially merge the alternate dimension with Gallifrey's Web of Time. (COMIC: Who's That Girl!)

At one point, travel between parallels was made relatively simple by the Time Lords — although TARDISes were still equipped with security protocols to stop them travelling into other universes on a regular basis. (TV: Inferno, PROSE: The Quantum Archangel) Transit between universes was initially very difficult in the post-War universe, but Sabbath Dei's scheme with Maxwell Curtis damaged the stability of the multiverse and left the Eighth Doctor unable to control which universe his TARDIS went to. (PROSE: Time Zero) According to the Tenth Doctor, travel to parallel universes became almost completely impossible after the Last Great Time War, and could only happen under extraordinary circumstances. (TV: Rise of the Cybermen)

Unbound Universe
In a parallel universe that Bernice Summerfield called the Unbound Universe, (WC: A Visit from Prof. Summerfield) an alternate Doctor was exiled to Earth in 1997, rather than the 1970s. After escaping his exile, he travelled with this universe's version of Alistair Lethbridge-Stewart, (AUDIO: Sympathy for the Devil) until an encounter with the Daleks where Alistair decided to stay on Skaro to help a fledgling peace between Daleks and Thals. (AUDIO: Masters of War)

After a major war across time and space, the Doctor was left the last of his kind, and thus the Ruler of the Universe, and the universe itself dying. After a failed attempt to flee to another universe which resulted in Bernice Summerfield becoming stranded with him, (AUDIO: The Library in the Body) and foiling a scheme by this universe's version of the Master, (AUDIO: The Emporium at the End) the Doctor reluctantly accepted his responsibilities and worked as President of the Universe to establish a safe zone using the Apocalypse Clock. (AUDIO: The True Saviour of the Universe)

This universe eventually collapsed into nothingness, with the only remaining inhabitant its version of the Master in a shielded bubble. After thousands of years of waiting, he was rescued by the Dalek Time Strategist. (AUDIO: Shockwave)

Alternate Earths
The Fifth Doctor visited a parallel Earth in which World War II was still being fought in July 1963. While there, he met Angus Goodman, who became his companion for a time. (COMIC: Lunar Lagoon, 4-Dimensional Vistas, The Moderator)

On one parallel Earth, an alternate Doctor helped the establishment of a fascist republic in Great Britain. PROSE: Timewyrm: Revelation, I, Alastair) In the 1970s, the Third Doctor of N-Space accidentally travelled to this Earth whilst repairing his TARDIS console, becoming embroiled in its version of the Inferno Project which devastated this Earth. (TV: Inferno) A group of survivors later attempted to invade N-Space, coming into conflict with UNIT and . (PROSE: The Face of the Enemy)

There were a number of parallel versions of Earth where the Roman Empire had never fallen. (PROSE: Warlords of Utopia) The Fourth Doctor encountered invaders from one parallel Earth in which the Roman Empire spanned the galaxy. (COMIC: Doctor Who and the Iron Legion, AUDIO: Doctor Who and the Iron Legion) On another such Earth by 1988 the Roman Empire owned three-fifth of Earth and Empress Margarita was Caesar, with America and Australia being populated by "savages". This Earth's version of Melanie Bush was a slave and like her N-Space counterpart travelled with the Sixth Doctor, who had lost one of his eyes. (PROSE:  Spiral Scratch) During the War in Heaven a number of these Roman parallel Earths united to form the Empire of Empires and came into conflict with Greater German Reich, a rival union of Earths where the Nazis had won World War II. (PROSE: Warlords of Utopia)

The Sixth Doctor and Evelyn Smythe travelled to a parallel Earth in which the Doctor actually enjoyed his exile to the planet. (AUDIO: The 100 Days of the Doctor)

Melanie Bush met a version of herself from a universe where Silurians never went into hibernation. In this universe, she was Melanie Baal, a Human-Silurian hybrid. (PROSE: Spiral Scratch)

On one parallel Earth, peace was established between humanity and the Silurians after the Wenley Moor incident, despite the Third Doctor being killed during the event. The peace was kept by the United Races Intelligence Command, a counterpart of N-Space's UNIT. (COMIC: Final Genesis)

When Sabbath's actions caused the barriers between various parallel realities to break down (PROSE: Time Zero), the Doctor visited several different Earths whilst trying to restore the prime reality, including a world where a group had been formed specifically to kill Sabbath (PROSE: The Infinity Race), a world where the computer was never invented as part of the efforts of that world's Sabbath to 'protect' his Earth (PROSE: The Domino Effect), a world where the mysterious Eternines sought to drain Earth's energy to save themselves (PROSE: Reckless Engineering), and a world where time-travel tours were founded while Earth faced invasion by an alternate race of Martians. (PROSE: The Last Resort)

In one parallel universe, an alternate Doctor regenerated into a female incarnation and hide on Earth in the 21st century in an attempt to evade the Time Lords who had sentenced her to exile. Her efforts to hide failed and she was recaptured. (AUDIO: Exile)

In 2007, the Tenth Doctor, Rose Tyler and Mickey Smith visited a universe in which Rose's father was still alive and the Cybermen were created on Earth by John Lumic. In this universe, the United Kingdom was instead the People's Republic of Great Britain led by the President of Great Britain. (TV: Rise of the Cybermen) The Torchwood Institute in this universe had managed to develop a technology that allowed quick travel between their dimension and the Doctor's World through the breach created in the Void by the Dalek Void Ship, but as an unintended side-effect, travelling in this manner further opened the breach, to a point where it started threatening the existence of that universe. When the breach was closed by the Doctor, this method of travel was rendered unusable. (TV: Doomsday)

On one parallel Earth, in 2008 Ianto Jones accepted the offer of Mairwyn. He did not die when the 456 invaded and Mairwyn enslaved Earth. The two later visited the primary universe to observe Ianto's funeral. (COMIC: Shrouded)

Donna's World was a "great big parallel world" created around Donna Noble by a Time Beetle, where the Tenth Doctor died facing the Racnoss under the River Thames in 2007, and was subsequently unable to save the people inside Royal Hope Hospital who were taken to the Moon in 2008, or the Earth from being attacked by a replica of the Titanic, the Adipose or ATMOS in 2008-09. When the Donna of this world travelled back in time to make her younger self turn left at a junction on Little Sutton Street, both the Doctor and Donna believed that this world ceased to exist. (TV: Turn Left)

In 2009, Rose used the dimension cannon created in Pete's World to search through the parallel universes to find the Tenth Doctor, seeking his assistance as the stars had begun to disappear. She visited numerous parallel Earths on this search. One was doomed to extinction when its own sun was extinguished. (AUDIO: The Endless Night) Another had been struck by extreme global warming believed to have been covertly caused by aliens and, despite a regression of technology in an attempt to slow it, it was predicted that humanity would become extinct within seventy years. (AUDIO: The Flood) On a third, Sir Pete Tyler founded SoulTech, which allowed the consciousness of deceased humans to live on in the soul machines. (AUDIO: Ghost Machines) In the fourth, the human race faced impending destruction at the hands of planetoid EK56. A common factor of these four universes, observed by Rose Tyler, was that there was no trace of the Doctor, or of the high-profile alien incidents which had threatened the Earth of the Doctor's World. (AUDIO: The Last Party of Earth)

One parallel Earth was ruled by a dictatorship and had been vistied by an alternate Doctor, who according to this Earth's Kate Stewart roamed the universe to annihilate "pathetic, pacifist aliens". This universe was inadvertently visited by Petronella Osgood and Josh Carter whilst studying a travel capsule UNIT had seized from the Auctioneers. They came into conflict with their far mre ruthless counterparts. (AUDIO: False Negative)

Expiermenting with more seized Auctioneer technology, UNIT discovered another parallel Earth which was found to be one of numerous realities which had been conquered by a force of Cybermen determined to hail neither from their own universe nor the parallel universe they were aware of. In a campaign to upgrade the entire multiverse, these Cybermen attempted to seize the Doctor's World starting with Earth only to be destroyed by. (AUDIO: Telepresence, Code Silver, Master of Worlds)

On one parallel Earth, the Doctor, who believed that the ends justified the means, foiled the secret experiments on the DEEP though became separated from his TARDIS. (AUDIO: Full Fathom Five)

In one parallel universe, Earth was part of a meta-government called the United Federation of Planets. This universe was joined with the Doctor's at one point, resulting in the Fourth Doctor coming in contact with the Federation in the 23rd century and the Eleventh Doctor in the 24th century, although this combination was undone when the Doctor and the Enterprise-D defeated the Borg-Cybermen alliance that caused this invasion. (COMIC: Assimilation²)

Alternate Daleks
Most versions of the Kaleds across the Multiverse became the Daleks. (AUDIO: Palindrome)

In one universe, the Daleks had been created as a force for good by the Mentor. (AUDIO: Dalek War: Chapter One) These Daleks ruled their universe and were contacted by the Dalek Emperor of N-Space via Project Infinity during the Second Great Dalek Occupation, who hoped to learn from their success. The parallel Daleks were horrified by the actions of their counterparts and declared war. (AUDIO: Project Infinity)

In the company of Romana II, the Fourth Doctor encountered a convergence of forces from three separate parallel universes whose respective dominant powers were the Quarks, the Kroton Imperium, and the Ogron Confederation of Planets, which he theorised to be the result of a hypothetical absence of the Daleks in their realities. (COMIC: The Lost Dimension) One parallel universe where Davros never created the Daleks was visited by the Dalek Time Strategist during the Last Great Time War, where he manipulated Davros into merging the Kaleds with echoes of their counterparts, turning them into Daleks, and uniting Davros himself with echoes of his other selves, providing a means of restoring the Daleks following their erasure in N-Space. (AUDIO: Palindrome)

Alternate Time Wars
In one parallel universe, the War in Heaven started when an alternate Sixth Doctor convinced his people to fight the Enemy, who were an old foe of his. In the course of this war, the Time Lords allied with the Cyberlords and the Doctor became their President. The tide of the conflict turned against the Time Lords after the Master betrayed them, eventually resulting in the Doctor using the Armageddon Sapphire to destroy this universe. (PROSE: The Quantum Archangel)

In the Warrior's universe, Sarah Jane Smith successfully convinced the Fourth Doctor to destroy the Daleks in the incubation chamber during his mission to avert their creation. (AUDIO: Dust Devil) His actions however led to the formation of the Unified Skaroan Alliance, who obtained time travel from his time ring. (AUDIO: Aftershocks) Mortally wounded by a surviving prototype Dalek, the Doctor was extracted from Skaro by Narvin, who informed the Doctor his actions had caused a Time War and provided him with the Elixir of Life, triggering his regeneration into the Warrior. (AUDIO: Dust Devil)

Meta-fiction universes
On two occasions, the Doctor encountered universes where he was a fictional character in a TV show called Doctor Who. (COMIC: TV Action!, The Girl Who Loved Doctor Who)

In one universe, Doctor Who was cancelled before its first episode even aired, much to the disappointment of the writer Martin Bannister. (AUDIO: Deadline)

Other parallel universes
In 1997, Carbury was invaded by knights from a parallel universe which bore similarities to Arthurian legend. (TV: Battlefield) The Eighth Doctor eventually returned to this world - before this invasion from their perspective - when he unintentionally disabled some of the TARDIS's security protocols, allowing him to set up the events that would lead to his past self thwarting their earlier invasion. (PROSE: One Fateful Knight)

One parallel universe was lifeless and empty due to the destruction of Logopolis in 1981. (PROSE: The Quantum Archangel)

In one parallel universe, an alternate Doctor was delayed leaving Gallifrey due to the machinations of Quences. After escaping due to the intervention of Susan, (AUDIO: Auld Mortality) this Doctor travelled freely in time and space, causing huge alterations to Earth's history in the process. (AUDIO: A Storm of Angels, PROSE: The Innocents)

In one universe, the Valeyard won the Doctor's trial and successfully obtained the Sixth Doctor's remaining regenerations. He went on to meddle in history, believing he was taking advantage of opportunities his other selves had squandered, but destabilised his own timeline in doing so. (AUDIO: He Jests at Scars...)

The Eighth Doctor was once shown many of his counterparts who existed in parallel universes across the Multiverse, including a version of his current incarnation who had married Grace Holloway, another version of his current incarnation who was a cyborg, Doc Gallifrey, Joe Smith, Theta Stigma, Quiquaequod and Tardis Tails. (COMIC: The Glorious Dead)

Whilst infected with anti-time, the Eighth Doctor claimed he saw "a tiny reality", where Gallifrey was not a planet, but a diamond that drifted through space. (AUDIO: Zagreus)

Forty-five billion universes to the left of the Doctor's universe was the universe home to the Word Lords. (AUDIO: The Word Lord)

"All thirteen" incarnations of the Doctor came together to save Gallifrey in the Last Great Time War by freezing it in a single moment in time, in a "parallel pocket universe". (TV: The Day of the Doctor)

Miscellany
Fitz Kreiner once told Trix MacMillan that "the future" was the best place to shop for music, since one could pick up an artist's complete run. Parallel universes, however, complicated this theory: "You think you've got all the Beatle albums until you've been to a few of those." When later called upon to sing in a bar, Fitz briefly considered using one of the "parallel universe Beatles songs," since no one in the prime universe would recognise them. However, when Trix pointed out that one of these songs was merely "Jealous Guy" with different lyrics, he quickly abandoned the idea of using these songs as "cheating." (PROSE: The Gallifrey Chronicles)

Chronovores feasted on parallel universes, but their meals were infrequent because they were banished to Calabi-Yau Space, which provided them no access to the timelines and parallels of the normal universe. once released the Chronovores to the Doctor's World, where they threatened to eat away all realities. (PROSE: The Quantum Archangel)

In all alternate universes with Melanie Bush, she grew up with her sister Anabel. In the prime universe, Melanie accidentally killed Anabel when they were children. (PROSE:  Spiral Scratch)

As colloquialism
The term was used sometimes in an inexact way, simply to indicate the unlikelihood of an event. Claudia Bruderbakker once told Peri Brown, for instance, that if her stepmother Joan Bruderbakker had been responsible for a particular event, she'd "entered a parallel universe". Peri, familiar with parallel universes, briefly considered the genuine possibility of another universe being created, but then realised that Claudia was speaking figuratively. (PROSE: Synthespians™)

Other references
Sarah Jane Smith corrected Maria Jackson when she referred to Andrea Yates' World as a "parallel world", explaining that it was their timeline having been changed. (TV: Whatever Happened to Sarah Jane?)

Upon seeing an elephant on the ice in London, Bill Potts believed that the TARDIS had arrived in a parallel world; it had infact landed in the 1814 frost fair. (TV: Thin Ice)