Rare metallic element that occurs naturally on some worlds, which is vital to Asgard technology. It is also vital to the construction of human-form Replicators, which are made up of trillions of microscopic nanites.
The new Asgard homeworld of Orilla was chosen because it was rich in neutronium.
New Order, Part 1 - The Asgard attempt to restart their civilization on the neutronium-rich planet Orilla, but the Replicators invade.
New Order, Part 2 - The Replicators are forced to leave Orilla, but not before Fifth has used neutronium to engineer a mate: a Replicator copy of Samantha Carter.