Web of Time

The Web of Time was the orderly co-existence of events within Time.

It was not a necessary fundamental aspect of the universe, but rather a meta-structure created and imposed on history by the Time Lords. It could not exist until Rassilon built the Eye of Harmony and used it as what has been described as "the hitching post of chronology." (AUDIO: Neverland)

The Web was not invulnerable; for example, the Sixth Doctor once explained that to destroy the planet Earth in 1985 would disrupt the Web of Time. Additionally, Lytton spoke of the destruction of Mondas in 1986 as "now" being a part of the Web, implying its destruction wasn't necessarily fixed. (TV: Attack of the Cybermen)