Mellon

The Elvish word for 'friend', made famous by its use as the magical password to the Dwarf-city of Khazad-dûm. When the Fellowship of the Ring arrived at the West-gate, Gandalf mistranslated the inscription as 'Speak, friend, and enter', and was thus unable to immediately fathom the opening spell. He tried various Elvish incantations, including:

This is translated by Christopher Tolkien (in volume VI of The History of Middle-earth) as 'Elvish gate open now for us; doorway of the Dwarf-folk listen to the word of my tongue.' Only after numerous other attempts (including the imperative Edro!, 'Open!') did it strike Gandalf that he had misconstrued the inscription, which actually contained the opening word, 'Friend'. On uttering the Elvish Mellon!, the West-gate swung open. Immediately after Gandalf led the Fellowship into Moria, the doors were destroyed by the Watcher in the Water, and so Gandalf was the last to use the ancient password Mellon to enter the halls of ancient Khazad-dûm.