Alternate timeline

An alternate timeline is a reality that diverges from the "true" timeline due to the actions of time travellers or other temporal phenomena. They are different from parallel universes in that they do not separate cleanly at the point of creation resulting in the new timeline destroying the old one. (PROSE: Imperial Moon)

Physics
Alternate timelines can 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 is possible if the traveller possesses something, such as the sigil ring, that allows them to change time streams. (PROSE: Goth Opera) Whether it is possible or not to travel to a timeline after it has been negated is unknown, though every point in time has its alternative (TV: Pyramids of Mars) and so every alternative may always exist.

They may diverge from points in history where there is temporal flucuation, but beings with enough power are capable of changing time enough to create them, such as Sutekh and the Chronovores. Because the universe only has a finite amount of mass and energy, the creation of an alternate timeline adversely affects the real timeline, causing chaos throughout the universe; releasing enough energy can destroy the timeline and set history back on its correct course. (PROSE: Blood Heat) This may explain why the effects of the reality bomb (TV: Journey's End) and Rassilon's Ultimate Sanction (TV: The End of Time) could be observed before either event had even occurred.

Though a timeline may be negated, memories of it could still be retained.