Divergent Universe

The Divergent Universe was a universe that had no concept of time. It was home to the Divergence and other species. (AUDIO: Zagreus)

President Romana forced the Eighth Doctor to go to the Divergent Universe after he had been possessed by Zagreus. Charley Pollard made a leap of faith and joined him on his journey.

History
Rassilon found the Divergence inhabiting our universe, and deduced they would eventually surpass the Time Lords in power. To prevent this, he used his new control over Time and Space to seal them away in their own reality. To keep them imprisoned to the End of Time, Rassilon installed a mobius loop which took all life out of that universe once every twenty to thirty millenia before regurgitating the universe in its original form. A side effect of this was the lack of Time as a linear concept within the universe -- cause and effect had no meaning because no matter what happened things would always return to how they had started.

Rassilon used the force of Anti-Time to craft a weapon capable of forever destroying the Divergence. He infected the Eighth Doctor with it, creating the Zagreus persona. However Rassilon's attempts to direct Zagreus failed and Zagreus hurled Rassilon into the Divergent Universe to directly confront his enemies. (AUDIO: 'Zagreus')

Soon after, the Doctor exiled himself to the same universe to control his Zagreus persona. Rassilon extracted Zagreus from the Doctor, and then used both personas to break the cycle of destruction and recreation. (AUDIO: The Next Life)

Description
Time in this universe flowed as we normally experience it, from one moment to the next, but travel back and forth across the time stream was not possible. The inhabitants also had no understanding of the concept of Time. (AUDIO: The Creed of the Kromon)

There were many planets in that universe, including the planet Bortresoye, which was divided into zones by the Divergence.

The story arc
The term Divergent Universe also describes the story arc for the Eighth Doctor in audio.

Broadly speaking its narrative begins in Storm Warning, the first Eighth Doctor audio story. This sets up the Doctor's transgression of the Laws of Time, which unleashes the Anti-Time he absorbs in Neverland.

The Time of the Daleks is also a narrative precursor; it establishes that the Daleks get caught inside the time vortex -- an important point of both Neverland, which acts as the prologue for the arc, and Terror Firma, which acts as an epilogue.

A strict definition of the arc, however, has it begin with the closing moments of Zagreus, as the Doctor and Charley exile themselves to the Divergent Universe, and continues through to the closing scene of The Next Life, when they return to the main Universe.

Other, later stories tie into the Divergent Universe arc, including C'rizz's departure in Absolution and Charley's departure in The Girl Who Never Was.