The Dwarf kingdom beneath the Misty Mountains in J.R.R. Tolkien’s works is Khazad-dûm.
Khazad-dûm was the greatest ancient realm of the Dwarves. It was built beneath the central Misty Mountains and became famous for its vast halls, mines, and wealth in mithril, the exceptionally valuable metal found there.
After the Dwarves awakened a Balrog while mining, they abandoned the city. In the Third Age it became known by the Elvish name Moria, meaning “Black Chasm” or “Black Pit” in Tolkien’s constructed language Sindarin. The Fellowship passes through Moria in The Lord of the Rings, where Gandalf confronts the Balrog.
Erebor is the Lonely Mountain, not Khazad-dûm. Moria is the later name for the same ruined realm, so “Moria” can be a defensible descriptive answer in some contexts; this question asks for the original Dwarvish name.