The word that opens the West-gate of Moria in J.R.R. Tolkien’s The Fellowship of the Ring is “Mellon.”
The West-gate stands beside the lake called the Sirannon, outside the underground realm of Moria. Its doors bear an inscription devised by the Elf Celebrimbor, who made the Doors of Durin in cooperation with the Dwarf Narvi. The inscription says that speaking the password will open the doors.
“Mellon” is an Elvish word meaning “friend.” Gandalf eventually realizes that the inscription’s instruction is not asking for a secret name but telling the speaker to say the word “friend.” The doors open, allowing the Fellowship to enter Moria.
The scene is often remembered for the contrast between a simple password and the difficulty of interpreting the inscription. After the Fellowship enters, the Watcher in the Water attacks and blocks the exit. Moria itself is also called Khazad-dûm, its name in the language of the Dwarves.