Adamantium, also known by similar names such as Adamantine, adminium, adamantite, etc., is a fictitious metal or alloy common in media that's generally held to be stronger and lighter than
titanium steel (titanium is far from being the strongest metal, and saying it is will trigger every engineer and chemist who hears you), if not outright completely indestructible. At other times it is stronger but heavier. Possibly high-carbon steel (in mythology, not fantasy). Vikings made high-carbon steel swords capable of cutting through shields, armor, and enemy swords and gave these magical blades to their greatest warriors. Remember, swords are piercing weapons, so imagine just how stupidly OP these things were if their mere cutting edge goes through steel plate.
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. The Worf of metals. In a way it seems to be one of those things where either a weapon is useless against it or adamantium is useless against the weapon. Nothing in-between.