Alternate timeline

An alternate timeline was a reality that diverged from the "true" 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) Alternate timelines constantly winked in and out of existence with every decision made. (AUDIO: The Wrong Doctors)

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. (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)

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 and the Doctor dying on Grace Alone and later Trenzalore. (PROSE: The Time Travellers, AUDIO: The First Wave, TV: The Name of the Doctor,The Time of the Doctor)

Metaphysics
From a metaphysical viewpoint, alternate timelines were 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 were held in check by the existence of the Time Vortex, which was sustained by the existence of the Time Lords. (PROSE: The Domino Effect)

Terminology
The phrase "alternate/alternative timeline/universe" was sometimes used interchangably 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.