Mithril is the fictional metal used for Bilbo’s coat of mail in J.R.R. Tolkien’s The Hobbit.
Bilbo receives the mail shirt from Thorin Oakenshield after the company discovers treasure in the dragon’s hoard. Although the shirt is made for a much larger person, it fits Bilbo and later protects him during dangerous encounters.
Mithril is described as exceptionally light, strong, and valuable. Its name combines the Sindarin words for “grey” and “glitter,” and the metal is especially associated with the dwarves’ mines of Moria.
The material is sometimes called “silver steel” in loose summaries, but that phrase is descriptive rather than the canonical answer. Mithril also becomes important in The Lord of the Rings, where Frodo’s shirt saves him from a severe blow in Moria.