Lydroxide was a corrosive chemical compound used as a coolant in the tether couplings of Nezu mag-lev carriages.
In 2372, the Etanian collaborator Sklar killed Doctor Vatm by poisoning his water with lydroxide. Sklar had uncovered evidence that the asteroids bombarding the Nezu planet were of artificial origin, threatening Etanian annexation plans. (VOY: "Rise")