In Stranger Things, what is the name of the parallel dimension?

In Stranger Things, the parallel dimension is called the Upside Down.

The Upside Down is presented as a dark, hazardous counterpart to Hawkins, Indiana. Its geography mirrors the ordinary town, but its buildings are decayed and covered with vines, spores and other organic growths. The dimension is inhabited by dangerous creatures, including the Demogorgon and Demobats, and is connected to Hawkins through supernatural gates.

The name comes from the young characters' Dungeons & Dragons-inspired way of understanding the supernatural world. In the first season, they use the phrase “the Upside Down” to describe a shadowy realm existing alongside their own. The term became one of the show's defining pieces of mythology.

Viewers sometimes confuse it with the Void, which is the black, empty psychic space Eleven enters during her sensory-deprivation exercises. The two locations are related to the show's supernatural mythology but are not the same place. The Upside Down is the series' recognizable parallel-world setting and a central source of its horror.