Monotanium was a metallic alloy typically used in the armor plating of Hirogen ships and space stations. It also had the added effect of scattering targeting beams. (VOY: "Hunters")
On their first encounters, USS Voyager's weapons were relatively ineffective against the Hirogen ships' armor. (VOY: "Prey")
An Entharan target buoy used to demonstrate Kovin's isokinetic cannon was made from ten meters of solid monotanium. (VOY: "Retrospect")