The Matrix

The Matrix was a domain within the APC Net (Amplified Panatropic Computer Network), a supercomputer used by the High Council of the Time Lords as a storehouse of knowledge and to predict future events. The terms APC Net and Matrix were often used interchangeably.

Uses of the Matrix
Returned to Gallifrey by Councillor Goth, the Master used of the Matrix, secretly infiltrating it and using Goth as his agent within it. Goth confronted the Fourth Doctor there, and attempted to kill him. (DW: The Deadly Assassin)

As Lord President, the Doctor used the Matrix to gain access to the secrets needed to defeat the Sontaran invasion of Gallifrey, specifically the De-mat Gun. (DW: The Invasion of Time) While connected to the Matrix, he learned of the existence of the Timewyrm, and left a message to himself warning of this threat. (NA: Timewyrm: Genesys)

After his supposed execution to stop Rassilon's exiled contemporary Omega from returning to the universe of matter, the Fifth Doctor hung suspended in the Matrix. Omega also had access to the Matrix. (DW: Arc of Infinity)

The Valeyard established a stronghold, the "Fantasy Factory", in the Matrix as part of his plan to steal the Sixth Doctor's remaining regenerations. During his attempt to stop him, the Doctor, as well as the Master, entered into the "dreamscape" therein, the latter taking his TARDIS (or an illusory version of it) there. The Valeyard somehow took over the Keeper of the Matrix. (DW: The Ultimate Foe)

The Seventh Doctor confronted the Dark Matrix, which was trapped inside a TARDIS as it imploded. (PDA: Matrix)

Fate of the Matrix
An ancient Gallifreyan evil named Pandora also survived and emerged from a special partition within the Matrix. Lady President Romana was eventually able to destroy the entity by destroying the Matrix itself. (BFG: Lies and others in series 2)

Another account said that Gallifrey was destroyed by the Eighth Doctor to prevent the voodoo cult Faction Paradox from starting a time war between the Time Lords and the Enemy. (EDA: The Ancestor Cell) It was later revealed that the Time Lords had survived within the Matrix, which had then been downloaded into the Doctor's mind, although he had to sacrifice much of his memory to make space for it. Their reconstruction was possible, but would require a sufficiently advanced computer. (EDA: The Gallifrey Chronicles)


 * This event may take place in an alternative timeline or in the future, relative to the other event, given that by this time, Romana had regenerated into a third incarnation. Possibly both they and the planet were restored at some point before Gallifrey's later destruction.

Characteristics
The Matrix, specifically, contained a simulated reality environment, once described as a "micro-universe" (DW: The Ultimate Foe) which stored the personalites of Time Lords now without physical bodies and even the past incarnations of living Time Lord. (IDW: The Forgotten) Living beings could also access the Matrix. The terms Matrix and APC Net, however, have sometimes been used interchangeably.

The Matrix stored the memories of dead Time Lords in a framework of electrochemical cells. (DW: The Deadly Assassin) It also received data from sensors in TARDISes. (PDA: The Quantum Archangel) It was not only a record of the past, but could predict the future as well. The amount of knowledge in the Matrix, though vast, was not complete, and could be tampered with, given access. The unauthorised extraction of a Time Lord's bio-data from the Matrix was an offence tantamount to treason. (DW: Arc of Infinity) A particularly skilled person such as the Valeyard could create images of events that never had happened nor ever would. (DW: Mindwarp, Terror of the Vervoids, The Ultimate Foe)

If two or more incarnations of the same Time Lord accessed the Matrix simultaneously, they all gained equal access to all other incarnations' memories. Because of this, a young, pre-Key to Time version of Romana I instantly understood her future when Lady President Romana II joined her in the Matrix. (BFG: Lies)

Environment
To living beings, the Matrix could appear like conventional reality (DW: The Deadly Assassin, The Ultimate Foe), a surreal dream (DWM: The Tides of Time), a dark void (DW: Arc of Infinity) or first one, then another. Physical laws were malleable to the will of the inhabitant. Experience and sheer will gave one control over "reality".

Beings known as the Matrix Lords, including Rassilon himself, "lived" there and could direct actions in the universe. They created a physical agent, Shayde, to act for them in the outside world. (DWM: The Tides of Time)

Physical access
Access to the Matrix was through an apparatus connected to the head of the user. (DW: The Deadly Assassin) The Crown of Rassilon worn by the Lord or Lady President gave them instant access. (DW: The Invasion of Time) The Keeper of the Matrix held the Key of Rassilon, granting access to the Seventh Door, thought legendary until the Doctor used the Key to access it. The Seventh Door allowed physical access to the Matrix. (DW: The Ultimate Foe)