Tylium is a fictional metallic ore featured in both versions of the science fiction series Battlestar Galactica. In both incarnations the substance is considered very rare throughout the known universe, but essential for fueling both human and Cylon space ships, including for the purpose of FTL jumps.
Tylium is featured in the original Battlestar Galactica's pilot, where it was pronounced "Tie-lium", and also in the re-imagining (where it is pronounced "till-ium", or occasionally "Tylinium") - specifically, episode 10, season one.
In the original 1978 pilot episode/movie, there are references to mining Tylium, which implies that it is a solid in its natural state; however, fuel consumption meters on the Viper instrument panel behave as if the fuel is liquid, so apparently this is a state change or a suspension to make metering and injection more reliable.
In the re-imagined series, raw Tylium ore appears as a yellow, sand-like substance before it is refined into a golden liquid similar in appearance to motor oil, however it flows quickly like gasoline. In the episode "Dirty Hands", it is shown that the raw Tylium is stored in massive silos aboard the fleet's Refinery Ship where it is poured (by hand) onto conveyor belts and heated in ovens. It was never specified how this process actually converts the raw Tylium to a liquid state, however it is stated that "hot" Tylium is highly unstable, and if the carefully controlled process is interrupted it could "go critical" and explode.
In the episode "Hand of God", Gaius Baltar mentions in dialog that the enthalpy of refined Tylium is . Tylium is unstable and has a strong exothermic reaction if sufficiently heated, especially the less stable refined Tylium precursor. It is also mentioned that Tylium will become useless if subjected to strong radiation, which renders the ore inert.