Ithildin was a magical substance made by the Elves out of the metal mithril. Ithildin could only be seen by the reflected light of the Moon and stars, and even then remained hidden until a magical word was said. The designs on the Doors of Durin were made from this substance.
Tolkien stated that ithildin is a Sindarin name, meaning "moon-star(light"), "moonlight" or "starlight". The word contains the elements Ithil ("moon") + tin/tîn ("spark; star; twinkle of stars"). He noted that the correct Sindarin form should be ithildim.