Charged Vacuum Emboitement

A Charged Vacuum Emboitement or CVE was a violent pathway between universes. CVEs provided a transit point between N-Space and E-Space. (TV: Full Circle)

Physics
Without mass, (AUDIO: The Quest of the Engineer) CVEs were the rarest of space-time events and almost undetectable, entities containing layers of highly charged quantum particles suspended in pockets of super-vacuums. They occupied the space between realities and were so violent in their physics that when they formed they punched a hole between universes, pushing space aside to insinuate themselves into reality, releasing massive waves of energy in the process. Passing through one was a disorienting experience. They had no external energy signature and made no detectable footprint in the three primary dimensions, yet could be as large as a moon or even a planet. (AUDIO: The Invasion of E-Space)

CVEs operated passively, drawing both physical objects and entropy through them. Because of their special function, the closure of the CVEs would threaten the physical integrity of N-Space and all life within. (TV: Logopolis)

The Doctor's universe took the energy it needed to continue expanding from pocket universes, drawing the energy through black holes or Charged Vacuum Emboitements. (PROSE: Dominion)

History
The Logopolitans created CVEs using Block Transfer Computations in order to save N-Space, their universe, the existence of which was threatened by entropy. It was the job of the Advanced Research Unit to keep the CVEs open. (TV: Logopolis)

While all complex systems eventually break down, the Logopolitans reasoned that if they opened a closed system (N-Space) into a smaller one (E-Space) they could extend the life of the larger one. By the year 1981, N-Space had already passed the point of no return into heat death long ago, the CVEs were the only thing keeping it still viable. Also that year, only one CVE (located in Cassiopeia) was left, eventually closing in the far future. (TV: Logopolis, PROSE:  Timewyrm: Apocalypse)

While in flight, the Doctor's TARDIS travelled through a CVE into E-Space by accident. (TV: Full Circle) The Fourth Doctor, Romana II, K9 and Adric wandered this universe for months, searching for another CVE to return them to N-Space. (AUDIO: The Invasion of E-Space) In the middle of the 22nd century, the human ship Hydrax passed into E-Space, likely through a CVE, though the King Vampire had some involvement in bringing them through. (TV: State of Decay, PROSE: Lucifer Rising)

Adric attempted to devise a computer program that would allow the TARDIS to travel back to E-space once it found a suitable CVE, but he died in a confrontation with the Cybermen before he could use it to return home. (TV: Earthshock) Unfortunately, the program he had written remained in the TARDIS computers, which activated when the TARDIS passed by another CVE after his death, trapping the Fifth Doctor, Nyssa, Tegan and Turlough back in E-Space. (AUDIO: Mistfall) The TARDIS was eventually able to travel back to N-Space through a final CVE, which was powered by Nyssa's own life energy. (AUDIO: The Entropy Plague)