Hell was a plane of existence or a universe believed by some cultures to be the place of punishment after death. Its opposite was Heaven. It was embedded in the minds of species like humans through religions like Christianity.
It had a fiery geometry that gave the universe its name. The Scourge lived here and tortured the humans of their universe. (BFA: The Shadow of the Scourge)
- Based on their statement, the Scourge either brought their victims to Hell, or there may be humans living in Hell.
Jason Kane spent several years in Hell, described as predominantly rock where tunnelling craft tunnelled their way between bubbles of places which weren't rock, which would in theory be the equivalent of planets. This may have been the Infernal Regions, however, which is probably where the idea of Hell came from. (BFBS: The Infernal Nexus)
The Tenth Doctor claimed that certain species would call the Void Hell. (DW: Army of Ghosts)
Hell was also a part of Gallifreyan mythology (BFA: Minuet in Hell) as mentioned by Morbius to describe his deteriorated state. (DW: The Brain of Morbius) The Tenth Doctor referred to the Last Great Time War as Hell. (DW: The End of Time)
