Mellon is the word meaning “friend” in the inscription on the Doors of Durin in Tolkien’s Middle-earth.
The Doors of Durin stand at the western entrance to Moria, also called Khazad-dûm. Their inscription is written in Sindarin, the Elvish language associated with the Grey-elves, and includes the instruction “Speak, friend, and enter.”
Gandalf initially interprets the instruction as a password to be spoken, but he later realizes that the door requires the speaker to say the Sindarin word mellon. Once the word is spoken, the doors open.
Mellon is not the name of the doors, Moria, or the password in a broader unexplained sense; it is the Sindarin word embedded in the riddle-like instruction. The scene appears in The Fellowship of the Ring and highlights Tolkien’s use of language as a key to plot and history.