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")
The Hirogen utilized a tylium-based power source for their ships and training facilities . (VOY: "Flesh and Blood")
Tylium was also a fuel used in both the 1978 and 2004 versions of Battlestar Galactica. (Star Trek Encyclopedia, 4th ed., vol. 2, p. 431)