A metal alloy used throughout the structure and systems of Starfleet vessels, which microscopic parasites from an asteroid consumed aboard the Enterprise-D, nearly destroying the ship.
Nitrium is at the centre of the technical plot in the episode Cost of Living. The Enterprise-D destroys an asteroid threatening Tessen III, and a cloud of glittering particles from the debris passes unnoticed into the hull.
They are metal parasites that feed on nitrium, which is found in replicators, life support, the warp drive and much else. Wherever they have eaten, a gelatinous residue is left behind and the system fails. Faults multiply until life support collapses and the crew lose consciousness.
Data, who does not need air, works out what the creatures want. He flies the ship to the Pelloris asteroid field, which is rich in the metal, where the parasites are drawn off the ship towards a richer supply and the crew are revived.