In Stranger Things, the dark parallel dimension is called the Upside Down.
The Upside Down is presented as a hostile, shadowy counterpart to Hawkins, Indiana, the fictional town where much of the series takes place. Its landscape mirrors familiar locations, including homes, streets and the Hawkins laboratory, but is covered with vines, spores and decaying organic growth. The atmosphere and creatures make it one of the show's central sources of horror.
The dimension first becomes central during the events of the first season, which begins in 1983. Eleven's psychic powers and the experiments at Hawkins National Laboratory are closely connected to the opening of a gate between Hawkins and the Upside Down. Will Byers is taken there by the Demogorgon, prompting the main search that drives the season's story.
The name also reflects the show's tabletop role-playing influence. The children use Dungeons & Dragons terminology to understand supernatural threats, including creatures such as the Demogorgon and Mind Flayer. The Upside Down is sometimes confused with those monsters, but it is the dimension itself, not a creature within it.