Tiamat's Dimension is an interdimensional realm occupied by the god Tiamat.
In the springtime, Dana Barrett was on the phone with her mother when she was whisked from her kitchen to Tiamat's Dimension and interrogated by Tiamat about the nature of her time as host of Zuul. Once Tiamat was done toying with Dana, she was back in her kitchen. After being lured to Dana's apartment, the Ghostbusters took the stairs to the roof and instead found themselves in her dimension. After they Crossed the Streams on Tiamat, the Ghostbusters found themselves back on the apartment roof. After Tiamat sensed Gozer's presence in Ray's mind, she crossed over from her dimension.
From her dimension, Tiamat used a well and observed the prime dimension Ghostbusters' meeting about their missing ghosts with teams from various parallel dimensions. Peter signaled the end of the meeting and the start of the retrieval missions with a warning to be careful since they don't know what they're walking into. Tiamat laughed and remarked truer words were never spoken.
Aside from a throne, the dimension is a void.