What was the original name of Moria in J.R.R. Tolkien’s Middle-earth?

The original name of Moria in J.R.R. Tolkien’s Middle-earth was Khazad-dûm.

Khazad-dûm was the greatest mansion and underground kingdom of the Dwarves. It lay beneath the Misty Mountains and became famous for its wealth, especially the valuable metal mithril. After the Dwarves awakened a Balrog, they abandoned the realm, and later peoples called the ruined and abandoned place Moria.

The Fellowship of the Ring travels through Moria after leaving Rivendell. In the Chamber of Mazarbul, they discover the record of Balin’s failed attempt to reclaim the mines. The Fellowship then encounters the Balrog, whom Gandalf confronts on the Bridge of Khazad-dûm.

Khazad-dûm is not another separate city from Moria. The two names refer to the same underground realm in different periods and linguistic contexts. Erebor is the Lonely Mountain, while Gondolin and Minas Tirith are entirely different strongholds.