"Planet Hell" was the nickname of an M-class planet located in the Delta Quadrant.
The planet, discovered by USS Voyager in 2372, was found to have high levels of trigemic vapors and was used by a reptilian-like species as a birthing-ground. After their offspring hatched, they used the vapors to feed themselves until they were picked up by their parents. (VOY: "Parturition")
Background
The name is an in-joke, referring to the set where the planet scenes were filmed. Stage 16 at Paramount Studios, used since the premiere of Star Trek: The Next Generation, earned its nickname thanks to the harsh conditions that could be simulated there.