Tylium was a substance used as a power source.

The primary reactor coils on Quarra processed more than 8,000 metric tons of tylium per second, with a 94% thermal efficiency. (VOY: "Workforce")

Hirogen warships utilize a tylium-based power source. (VOY: "Flesh and Blood")

Tylium was also a fuel used in the 1978 version of Battlestar Galactica as well as its reimagining. (Star Trek Encyclopedia (4th ed., vol. 2, p. 431))