A tough metal alloy from which the Hirogen build their ship hulls and body armour, able to scatter targeting beams and making their hunters very hard to stop.
Monotanium is the material the Hirogen favour for both starship hulls and the heavy armour their hunters wear. It is strong enough to shrug off a good deal of weapons fire and it diffuses sensor and targeting beams, which makes Hirogen vessels awkward targets.
The alloy is identified in the Voyager episodes Hunters and Prey, when the crew first meet the species and find that their weapons have trouble locking on to the heavily plated ships closing in on them.
Monotanium fits the Hirogen way of life: a nomadic people who live aboard their ships and spend generations pursuing dangerous prey need equipment that lasts and that can take punishment from whatever they corner.