Ithildin was a magical substance made by the Elves from mithril, that 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.