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) This probably allowed Rassilon to choose what species developed, as he did when he inprisoned the Divergence, a possible rival to the Time Lords. (AUDIO: Zagreus) While not a fundamental universal aspect, the Web maintained a balance which reflected the holistic nature of Time. (TV: The Two Doctors)
This balance could be upset, as 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)
Preventing disasters could cause others to take their place. Because of the Titanic sinking, all later ships would have round-the-clock radio surveillance, enough lifeboats, and the International Ice Patrol would be formed. Saving the Titanic would condemn the other ships to follow. (PROSE: The Left-Handed Hummingbird) If Adolf Hitler was prevented from rising to power, a more competent man would have taken control of the Nazi party and World War II would've ended differently. (PROSE: Timewyrm: Exodus)
The Doctor would interfere with the Web in different situations. When his friend Remnex was murdered by a dagger through the eye, the Seventh Doctor rewrote history so that he died peacefully in his sleep instead and was never possessed by Vilus Krull. (AUDIO: The Dark Flame)
Events that were part of the Web included:
- The destruction of Mondas. (TV: Attack of the Cybermen)
- The sinking of the Titanic. (PROSE: The Left-Handed Hummingbird)
- The destruction of the Halcyon through the Ice Warriors creating an atmosphere on Halcyon more suited to them. (AUDIO: The Resurrection of Time)