The Web of Time or Spiral Politic, also called the Kaliyuga, was the orderly structure of time, built and maintained by the Time Lords. It was Gallifrey's noosphere.
Overview
Rather than being a necessary fundamental aspect of the universe, the Web of Time was a meta-structure created and imposed on history by the Time Lords (AUDIO: Neverland, PROSE: The Book of the War) during the anchoring of the thread, (PROSE: The Book of the War) when Rassilon used the Eye of Harmony as "the hitching post of chronology". (AUDIO: Neverland) Before this, the universe had no history or defined set of physical laws. (PROSE: Christmas on a Rational Planet)
The Web could become unbalanced by the changing of events that had significant effects. For instance, the Sixth Doctor once explained that the Web of Time would be disrupted if Earth were destroyed in 1985, (TV: Attack of the Cybermen) as Earth was a nexus world linked to the destinies of many major intergalactic powers. The saving of the Titanic would have similarly perturbed the Web by condemning all the later ships that were saved by round-the-clock radio surveillance, lifeboat capacities, or the International Ice Patrol, all of which were developed due to the Titanic's sinking. (PROSE: The Left-Handed Hummingbird)
Even concerning events that were not fixed points, time could be crystallized along a particular web-way, (PROSE: The Sands of Time) making that event a fixed part of the Web of Time so that it had always happened and would always happen. (PROSE: Attack of the Cybermen) This could occur when a person learned about their personal future. (PROSE: The Sands of Time, TV: The Angels Take Manhattan) This effect was first predicted by Blinovitch. (PROSE: The Sands of Time)
Homeworlders were directly linked to their noosphere. (PROSE: The Book of the War) The Tenth Doctor explained to Donna Noble that only Time Lords could tell the difference between fixed points in time and events that were in flux. (TV: The Fires of Pompeii) The structure of the universe, this "web of space and time", was in constant flux as "an ocean" of shifting possibilities, only frozen into certainty through a Time Lord's direct touch or intervention. It was for this reason that the Time Lords established their non-interference policy. (PROSE: The Gallifrey Chronicles)
An imbalance in the Web could cause disastrous repercussions in every corner of the universe, killing billions, instantly disappearing major civilisations, and threatening the fabric of the universe itself. (PROSE: Attack of the Cybermen) However, an amaranth could be used to rebuild unstable or discontinuous elements of the universe (PROSE: Christmas on a Rational Planet) along the lines of the established laws of the Spiral Politic. (PROSE: Against Nature)
Specific events could sometimes be removed from the Web, but analogous events would take their place. (PROSE: The Left-Handed Hummingbird) For instance, if Adolf Hitler was prevented from rising to power, a different man would have still taken control of the Nazi party and begun World War II, though the War would've ended differently. (PROSE: Timewyrm: Exodus) The Seventh Doctor once rewrote history so that his friend Remnex died in his sleep rather than being murdered by a dagger through the eye. (AUDIO: The Dark Flame)
The Spiral Politic did not encompass the entire universe. Like with any noosphere, the Great Houses had great difficulty penetrating or even observing some areas around or beyond the Spiral Politic's edges, collectively known as the frontier in time. The Anvil Stars region was near this frontier in time, as were the planets Zo la Domini and Simia KK98 and the community Ascension. (PROSE: The Book of the War) The Tenth Doctor said that the Time Lords never endeavoured as far into the future as the year 100,000,000,000,000; (TV: Utopia) the Eighth Doctor's TARDIS would only take him as far forward as Endpoint, and then it needed to recalibrate before it could dematerialise again. (PROSE: Hope)
Events in the Web of Time
Events the Doctor stated as being 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 race through the Ice Warriors creating an atmosphere on Halcyon more suited to them, (AUDIO: The Resurrection of Mars) Deimos becoming an artificial sun that heated Mars, (AUDIO: The Resurrection of Mars) Gallifrey's Racnoss Wars, (AUDIO: Empire of the Racnoss) the burning of Joan of Arc, the assassination of Francis Ferdinand, (AUDIO: Cobwebs) the Manussan Empire overthrown in Manussan year 2326, (AUDIO: The Cradle of the Snake) Magnus Greel dying in the 1800s, (AUDIO: The Butcher of Brisbane) and humanity's 50-year war with the Eminence. (AUDIO: The Death of Hope)
Events in the Web that were explicitly recorded in the Matrix included the R101 crashing and all passengers dying, Adolf Hitler becoming Chancellor of Germany (and being alive to continue World War II), Edward VIII abdicating the British throne, German forces invading Poland, the evacuation of Dunkirk, Peladon entering the Galactic Federation, Zephon overthrowing the Embodiment of Gris, Mavic Chen being elected Guardian of the Solar System, Dalek forces assembling on Kembel and being engaged with the Movellans in 4-X-Alpha-4 in 4949, the Cult of Morbius being established, plans for the Doomsday Weapon being stolen by the Master, Chancellor Goth visiting Tersurus, President Romana and Etra Prime being taken by the Daleks and the subsequent Dalek invasion of Gallifrey being repelled by Romana, and a Dalek time fleet being captured in the Time Vortex. (AUDIO: Neverland) Charley Pollard was added to the Web when she saved it, not having died in the crash of the R101 as had been recorded. (AUDIO: Storm Warning, Neverland)
References
The Sixth Doctor believed summoning a Chronovore onto 21st century Earth threatened the Web of Time. (PROSE: The Quantum Archangel)
During the first battle of the War in Heaven on Dronid, the Great Houses discovered that the enemy had constructed their own meta-structure of the universe to replace the Spiral Politic. (PROSE: The Book of the War)
The Celestial Intervention Agency-operated Klyst Institute arranged to recruit their test subjects at the end of their lives in order to preserve the Web of Time. (AUDIO: Unregenerate!)
Since the Yssgaroth emerged during the creation of the Spiral Politic, the Yssgaroth taint remained in the fabric of the universe, irremovable except through the unweaving of time itself. The Book of the War speculated that the Yssgaroth were areas of hostile anti-structure produced by the collision of the Spiral Politic with the Spiral Yssgaroth's foreign and hostile structures and protocols. (PROSE: The Book of the War) Lolita thought that the Vampire Wars had dictated the shape of the Spiral Politic for the next ten million years. (PROSE: Toy Story)
A backup of the Spiral Politic was stored in the Great Houses' prison, which stood outside of time so that it could be remapped onto the universe if time was ever altered. (AUDIO: A Labyrinth of Histories)
Iris Wildthyme derisively called it the "Tale Without Meaning". (PROSE: First Meetings)
Ancient Gallifreyan prophecies recorded in the Matrix forewarned that the Hybrid, standing in the ruins of Gallifrey, would proceed to unravel the Web of Time. (TV: Hell Bent)