Khazad-dûm was the ancient name of the dwarf-city later known as Moria in J.R.R. Tolkien’s legendarium.
Khazad-dûm was founded beneath the Misty Mountains by Durin the Deathless. It became the greatest of the dwarf realms, famous for its mines, halls, wealth, and the discovery of mithril. The name means “Dwarrowdelf” or “Dwarf-delving” in Tolkien’s representation of the Dwarves’ own language.
The Dwarves eventually awakened a Balrog deep beneath the mountains while mining mithril. After Durin VI and his son were killed, the surviving Dwarves abandoned the city. Orcs later occupied it, and Elves came to call the ruined place Moria, a name associated with darkness and danger.
Erebor, or the Lonely Mountain, is a different dwarf kingdom, central to The Hobbit. Gundabad and Nogrod are also distinct locations. Moria is therefore the later, widely known name for the underground realm whose original Dwarvish name was Khazad-dûm.