The Other Side was a supernatural dimension where the spirits of dead supernatural beings resided in a state of purgatory or limbo. It was created by Qetsiyah with the intention of forcing Silas to spend an eternity with her on the Other Side instead of moving on and finding true eternal peace with his soulmate, Amara.
The Other Side was first mentioned by Anna when she appeared as a ghost and talked to Jeremy Gilbert, who had recently gained the ability to see ghosts after his resurrection. Anna told him she was all alone on the "other side;" though, Anna admitted that she did not know what it was officially called. However, other residents of the supernatural dimension, such as Sheila Bennett and Esther, referred to it as "the Other Side", a name that ultimately stuck and was used by the rest of the characters. This place was ruled by The Spirits.