Anti-time

Anti-time was a substance theorised by the Time Lords that would break down the Web of Time. The Antiverse was composed of it.

Description
Just as matter was opposed by antimatter, and every action had its opposite reaction, the Web of Time — positive time — must have had its negative counterpart.

The Eighth Doctor described it thus:

Time Lord theories suggested that anti-time was created when Rassilon created the Eye of Harmony, binding the universe to a single continuity. (AUDIO: Neverland)

History
At some point prior to Sutekh's civil war, the Osirians knew of anti-time. In order to sever him from the Great Houses' control, the Osirians inoculated Apep with a battert of alter-time states which included anti-matter, Schrödinger's Virus and anti-time. (PROSE: Going Once, Going Twice)

The theory of anti-time's existence remained discredited among the Time Lords until a series of anachronisms started wreaking havoc on the Web of Time. The Time Lords traced the source to the Eighth Doctor's companion Charley Pollard, who was a living paradox due to being saved from her death on the R101. Using her as a conduit, Madam President Romana and CIA Coordinator Vansell piloted a Time Station through her to the Antiverse: a universe made of anti-time inhabited by Neverpeople.

The Neverpeople attempted to destroy Gallifrey using a bomb filled with anti-time. The Time Lords on the ship were infected by anti-time, and set the Time Station to explode. However, the Eighth Doctor saved Gallifrey by materialising his TARDIS around the Time Station, thus containing the anti-time explosion. The Doctor and his TARDIS absorbed the anti-time energy released. Both were "infected" by it, and took on the persona of Zagreus. (AUDIO: Neverland)

Zagreus was courted by Rassilon to become his assassin. The TARDIS' outer shell was melted down in Rassilon's Foundry, and Zagreus formed a blade of anti-time, with which Rassilon hoped the Doctor would cut the Divergence from time. With the help of Charley and the TARDIS' "good side", the Doctor managed to reassert his personality long enough to refuse the offer and hurl Rassilon into the Divergent Universe. The infection was stabilised with pure zero matter. The Doctor, however, was forced to travel into a Divergent Universe to keep the Anti-Time and the Zagreus personality under control. (AUDIO: Zagreus)

The anti-time was eventually purged from the Doctor's system by Rassilon. (AUDIO: The Next Life)

Anti-Tempo Антивремя