Divergent Universe

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

President Romana forced the Eighth Doctor to go to the Divergent Universe without his TARDIS after he had been possessed by Zagreus. Charley Pollard made a leap of faith and joined him on his journey.

The Divergent Universe was created when Rassilon sealed away the Divergence into their own alternate as they would have ultimately surpassed even the Time Lords in power. Rassilon then sealed the Universe off completely by putting it in a mobius loop that took the form of a small blue planet which took all life out of that Universe once every twenty to thirty millenia before regurgitating it in its original form. This cycle was the reason time could not exist because cause and effect had no meaning as it whatever happened things would always return to how they had started. The cycle was corrupted when new elements arrived in the form of the Doctor, Charley and Zagreus. (BFA: 'Zagreus')

There were planets in that universe, including Bortresoye, which was divided into Zones. Two were Eutermes and Interzone. These were home to the Eutermesans and the Kromon respectively. C'rizz, who joined the duo on their travels, came from this planet. (BFA: The Creed of the Kromon) He went into the normal universe when the Doctor and Charley returned. (BFA: Terror Firma)

Rassilon was trapped in the Divergent universe after he removed the Divergence from the main Universe. (BFA: The Next Life)

The story arc
The term "Divergent Universe" also describes a story arc for the Eighth Doctor in audio. Broadly speaking its narrative begins in Storm Warning, the first Eighth Doctor audio. This sets up the Doctor's transgression of the Laws of Time, for which he must atone in Neverland. The Time of the Daleks is also a narrative precursor; it establishes that the Daleks get caught in the time vortex, an important point of both Neverland, which kicks off the Divergent Universe arc, and Terror Firma, which ends it.

A strict definition of the arc, however, has it begin with the closing moments of Zagreus, and continue through Scherzo — the first story completely set in the Divergent Universe — The Creed of the Kromon, The Natural History of Fear, The Twilight Kingdom, Faith Stealer, The Last, Caerdroia and The Next Life.

Terror Firma opens with the Doctor and his companions in the normal universe, but — like Zagreus, it's more a bookend than a proper part of the series. Other, later stories obviously tie into the Divergent Universe arc — such as C'rizz's departure in Absolution, and Charley's parting with the Eighth Doctor in The Girl Who Never Was —but the arc proper is over by the time the Eighth Doctor first confronts Davros in the opening scenes of Terror Firma, itself a cliffhanger resolution from the end of The Next Life.