Tylium (pronounced "TIE-lee-um") is an unstable mineral element used as fuel by both the Colonial and Cylon forces in the Seventh Millennium. The substance is often mined and it can explode when energy, such as discharges from laser pistols, impact its natural form.
Following the Battle of Cimtar, the Fleet heads to the planet Carillon, believing it to have tylium based on a report from an old mining expedition. Much to the surprise and shock of many, Carillon is discovered to have tylium in mass quantities, so much so that when the Colonials ignite the tylium with their energy weapons during a pitched battle with Cylon forces, the planet is destroyed in a cataclysm (TOS: "Saga of a Star World").
The Colonials make use of a separate fuel source, solium, for fueling various ships from Vipers to freighters, like Gemini (TOS: "Saga of a Star World", "Fire in Space"). This implies that tylium may well be the raw mineral from which other grades of fuel are derived, in the same way that diesel and gasoline are petroleum derivatives.
In the separate continuity
- Deathtrap!, which adapts the third part of Original Series' "Saga of a Star World".
- Battlestar Galactica Annual 2014, where Baltar's backstory is relayed, in addition to his discovery of Carillon.
- Battlestar Galactica Classic, where tylium is a rarity and source of contention within the Empyrean Expanse.