Web of Time

The Web of Time is or 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." (BFA: 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. (DW: Attack of the Cybermen)

Since the Daleks proved capable of manufacturing their own Eye of Harmony (BFA: The Time of the Daleks), they may also have been able to set up a hostile Web of Time during the Last Great Time War.

In the Post-Gallifrey universe, the Web may have gone the way of its creators, as history was now constantly in a state of flux and revision (DW: The Unquiet Dead). Worlds might quite plausibly be destroyed before their time, (DW: The Shakespeare Code) and whole eras of history, such as the Fourth Great and Bountiful Human Empire (DW: Bad Wolf), and the later two terms of Harriet Jones, briskly unwritten. (DW: The Christmas Invasion)

The stable fixity of the Web of Time appeared to have given way to a more mutable and dangerous "Timey-wimey" structure. (DW: Blink)

The state of flux, possibly caused by the absence of the Web, allowed for time to be rewritten (DW: Flesh and Stone) due to cracks in the universe caused by the explosion of the Doctor's TARDIS. (DW: The Pandorica Opens)

It is unknown whether the Web of Time, being near-exclusively referenced by novelists for works of unknown canonicity, is in use as a factor of lore in the current TV series. However, the Doctor has mentioned that since Gallifrey's destruction, time and space are more vulnerable to change and paradox. (DW: Father's Day (TV story))