Some or all of of a character's skin is made of metal. A common feature of many cyborgs and androids that aren't designed to look convincingly human, but it may also be the case with non-mechanical beings such as certain golems or elementals.
Respect the Tagging Guidelines and don't use this tag for characters that canonically have metal skin unless it actually appears to be made of metal. Seams, overlapping plates, and a lustrous finish are valid reasons to use this tag. Skin that is simply grey or yellow is not; that's what the grey skin and yellow skin tags are for. Shiny coverings that aren't actually metal might be scales or a carapace.