Alternate timeline

An alternate timeline was a reality that diverged from the "true" or "natural timeline" due to the actions of time travellers or other temporal phenomena. They were different from parallel universes in that they did not separate cleanly at the point of creation, resulting in the new timeline destroying the old one. (PROSE: Imperial Moon) The phenomenon may also have been described as an "interstitial spillage of time." (AUDIO: Project: Lazarus)

Physics
Alternate timelines could be visited accidentally by jumping a time track (TV: The Space Museum, AUDIO: The Mutant Phase) or contacting other anomalies (AUDIO: Colditz) in the Time Vortex. Deliberate travel to these timelines was possible if the traveller possessed something, such as the sigil ring, that allowed them to change time streams. (PROSE: Goth Opera)

They may have diverged from points in history when there was temporal fluctuation. The power required to adjust the timeflow and overcome temporal inertia on such a scale couldn't fit on a single planet, but beings with enough power were capable of doing so, such as Sutekh, the Guardians of Time, Rassilon, and the Chronovores. Even the Doctor or the Master could have the know-how or inclination to create them. Because the universe only had a finite amount of mass and energy to exist as one universe, the creation of an alternate timeline adversely affected the real timeline, causing chaos throughout the universe; releasing enough energy could destroy the timeline and set history back on its correct course. The energy released when a TARDIS time rammed another could produce this energy. (PROSE: Blood Heat)

Though a timeline could be negated, memories of it could still be retained. (TV: The Big Bang, The Impossible Astronaut) Sometimes these memories were subconscious and only surfaced in dreams. (AUDIO: The Time of the Daleks, Jubilee) Multiple timelines could co-exist in one continuum; because of this, an individual with multiple futures could come in contact without any Blinovitch Limitation Effects. (AUDIO: Peri and the Piscon Paradox)

Occasionally, time was actually changed so that what was originally the future became an alternate timeline. These were not meant to replace the true timeline but were the result of changes to the real timeline including, apparently, WOTAN conquering Earth (PROSE: The Time Travellers) and the Doctor dying on Grace Alone (AUDIO: The First Wave) and later Trenzalore. (TV: The Name of the Doctor, The Time of the Doctor) This may have related to causality, with knowledge coming before an event rather than an event becoming knowledge. The Doctor encountered WOTAN after it'd conquered Earth in 1966, changing history when he later arrived in 1966; the opposite would be the Doctor arriving in an 1804 where the French Revolution was still happening, as he already knew this was incorrect. (PROSE: The Man in the Velvet Mask)

Two Earth Reptile temporal scientists theorised that after changes were made to history, it took twenty minutes for the new timeline to become established. (PROSE: The Crystal Bucephalus)

Metaphysics
From a metaphysical viewpoint, alternate timelines constantly winked in and out of existence with every decision made, (AUDIO: The Wrong Doctors) always being extinguished as fewer and fewer choices became open. This was particularly affected by time travellers who changed the future whenever they arrived in a new time. This was the cosmic pain of the universe and in this manner Gabriel and Tanith were personifications of the timelines that never existed. (PROSE: Falls the Shadow)

The existence/creation of alternate timelines and parallel universes was held in check by the existence of the Time Vortex, which was sustained by the existence of the Time Lords who created it. (PROSE: The Domino Effect)

Terminology
The phrase "alternate/alternative timeline/universe/reality" was sometimes used interchangeably with "parallel universe"; however, alternate timelines usually changed after a single moment in time, whereas parallel universes usually had more than one difference from the "normal" universe.

Species from alternate timelines
Some species existed entirely as a result of diverges in time, including the Haemovores, a result of Fenric poisoning humanity; (TV: The Curse of Fenric) the Mind, before being destroyed by Gabriel and Tanith; (PROSE: Falls the Shadow) the Daleks affected by the Mutant Phase - an evolution of Daleks; (AUDIO: The Mutant Phase) and the Skaro Degradations, Dalek permutations from other timelines. (PROSE: Engines of War)

Terminology
The phrase "alternate timeline" is used in The Ancestor Cell, Beige Planet Mars, The Eye of the Giant, Just War, Legacy, Mad Dogs and Englishmen and The Shadow of Weng-Chiang. The phrase "alternative timeline" is used in Blood Heat, The Blue Angel, Cold Fusion, The Eight Doctors, The Eyeless, Imperial Moon, The Ripple Effect, Spiral Scratch and The Temptation of Sarah Jane Smith.