In J.R.R. Tolkien’s Middle-earth, “mellon” is the Sindarin word for “friend.”
The word appears on the Doors of Durin at the western entrance to Moria. The inscription instructs travelers to speak the password, and the solution is to say “mellon,” allowing the doors to open. The scene makes a vocabulary detail central to the Fellowship’s journey.
Sindarin is one of Tolkien’s principal Elvish languages. It is associated especially with the Elves of Middle-earth and was designed with a sound and historical development influenced partly by Welsh. Quenya is another major Elvish language, but “mellon” is Sindarin rather than Quenya.
The word is also used as the name of a fictional plant in Tolkien’s legendarium: mallorn trees are associated with Lothlórien. That related-looking term should not obscure the specific linguistic answer required by the Doors of Durin episode.