In Tolkien’s languages, the Sindarin word “mellon” means “friend.”
The word appears in The Fellowship of the Ring as the password for the Doors of Durin at the entrance to Moria. Gandalf eventually realizes that the instruction “speak, friend, and enter” requires the speaker to say the word mellon. The doors then open for the Fellowship.
Sindarin is one of Tolkien’s principal Elvish languages. He based aspects of its sound and historical development on Welsh, while Quenya was influenced in part by Finnish and Latin. Tolkien designed these languages with internal histories, vocabulary, and grammatical relationships rather than as collections of isolated fantasy words.
“Mellon” is sometimes described simply as an Elvish word, but the more precise classification is Sindarin. Quenya, another major Elvish language, uses a different word for “friend,” so identifying the language matters when discussing Tolkien’s vocabulary.