Parallel universe

A parallel universe was a reality separate from the main timeline but with similar aspects. Travel to parallel universes became impossible after the Last Great Time War. (TV: Rise of the Cybermen)

Origins
The origins of parallel universes is unclear. One theory is that they were created when historical events diverged from the main timeline naturally, as opposed to alternate timelines which involved a person changing the timeline deliberately. Still, the two terms were used interchangeably.

Another theory stated that parallel universes were simply that: parallel. They weren't "created" and had always existed. How similar they were to the main universe depended on how "far" they were from it.

Features
Parallel universes were so called because they bore similar aspects to the main universe of N-Space. These included humanoids and the same historical events, albeit usually with different outcomes. The term "perpendicular universe" also existed to apply to a universe that was radically different: E-Space was considered somewhere between a parallel and perpendicular universe.

Whereas alternate timelines generally existed in the same "time" as the main universe, time behaved differently in parallel universes. The Inferno Project had advanced more in one universe than it had in N-Space (TV: Inferno) and Pete's World was three years ahead of N-Space (TV: Journey's End)

Examples
In July 1963, the Fifth Doctor visited a parallel universe in which World War II was still being fought. While there he met Gus Goodman, who became his companion. (COMIC: Lunar Lagoon, 4-Dimensional Vistas, The Moderator)

In the 1970s, the Third Doctor visited a parallel universe in which the United Kingdom was a fascist Republic ruled by an alternate Doctor. (TV: Inferno, PROSE: Timewyrm: Revelation)

Circa 1979, the Fourth Doctor visited a universe in which the Roman Empire spanned the galaxy and was beginning to conquer other universes. (COMIC: The Iron Legion)

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

In the 1990s, Carbury was invaded by knights from a parallel universe which bore similarities to Arthurian legend. (TV: Battlefield)

In 2007, the Tenth Doctor, Rose Tyler and Mickey Smith visited a universe in which Rose's father was still alive. (TV: Rise of the Cybermen)

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 "real" universe to observe Ianto's funeral. (COMIC: Shrouded)

A parallel universe, where Earth was part of a meta-government called the United Federation of Planets, 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. (COMIC: Assimilation²)

In one parallel universe, the Ironsides were used to end World War II some years early, then were destroyed. (DWAN: Doctor Who The Official Annual 2011)