Adamantium, also known by simmilar names such as Adamantine, adminium, adamantite, etc., is a fictious metal or alloy common in media that's generally held to be stronger and lighter than titanium, if not outright completely indestructible.
Adamantine is one of several magical metals that have appeared in Dungeons & Dragons and, alongside Mithril, is one of the most iconic. Characterized by its jet-black coloration, adamantine is known for being super-durable; weapons made of the stuff can cut or smash through just about anything, whilst armor & shields made from it are virtually unbreakable.
The metal takes its name from a legendary ultra-durable metal in Greco-Roman mythology.
Adamantium is the Khaine of materials in 40K, in that it is literally only ever mentioned or discussed in terms of how a given weapon can effortlessly cut or smash through it.