Mellon is the password that opens the Doors of Durin in J.R.R. Tolkien’s The Fellowship of the Ring.
The word mellon means “friend” in Sindarin. The Doors of Durin stand on the western entrance to Moria, beside the lake near the Elvish realm of Eregion. Their inscription gives a clue that the visitor should speak a password, but the wording leads Gandalf to interpret “speak, friend, and enter” incorrectly for a time.
Gandalf eventually realizes that “friend” is the password itself. When he says mellon, the doors open. The scene demonstrates both the relationship between the Elves of Eregion and the Dwarves of Khazad-dûm and the way Tolkien makes language central to a puzzle.
The word is sometimes confused with an English translation or with a spell. It is neither: mellon is a Sindarin noun meaning “friend.” The doors then become a dangerous threshold, because the Fellowship encounters the Watcher in the Water outside Moria.