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)

Physics
Alternate timelines could be visited accidentally by contact with a time track (TV: The Space Museum, AUDIO: The Mutant Phase) or 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 where there was temporal fluctuation, but beings with enough power were capable of changing time enough to create them, such as Sutekh and the Chronovores. Because the universe only had a finite amount of mass and energy, 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 may be negated, memories of it could still be retained. (TV: The Big Bang, The Impossible Astronaut)

Metaphysics
From a metaphysical viewpoint, alternate timelines were always being extinguished as fewer and fewer choices became open. This is 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)