Deuterium
From Memory Alpha, the free Star Trek reference.
Deuterium (or heavy hydrogen) is an isotope of hydrogen which contains one proton and one neutron in its nucleus (or deuteron), and one electron orbiting the nucleus. Naturally-occurring hydrogen consists of less than 1% of this isotope.
Deuterium, along with its antimatter opposite, antideuterium, is the primary fuel used in matter-antimatter reactions, such as those generated in warp cores, and was stored in large tanks aboard starships for this purpose. (TNG: "Heart of Glory", "Captain's Holiday", "Final Mission", "Night Terrors", "Unification I", "Relics", et. al.)
The Bussard collectors on some starships were used to collect hydrogen and interstellar gasses, including deuterium. (TNG: "Samaritan Snare"; VOY: "Unforgettable", "The Haunting of Deck Twelve") However, the USS Voyager occasionally needed to find deuterium from other sources. (VOY: "The Cloud", "Demon")
- The reason for this remains unstated. It may be that the Bussard collectors or other processing systems were malfunctioning, or that the collectors cannot provide for 100% of a ship's needs, or that the Voyager was regularly using more deuterium than could be held in the tanks due to their constant use of the warp drive.
In 2371, Miles O'Brien and Jake Sisko investigated the possibility of turning the ore processing section of Deep Space 9, formerly used for processing uridium using Bajoran slave labor, into a deuterium refinery. They accidentally triggered Dukat's counter-insurgency program while doing so, however. (DS9: "Civil Defense")
The "Silver Blood", native to a Y-class planet in the Delta Quadrant, is a biomimetic lifeform composed of deuterium, hydrogen sulfate, dichromates, and proteins. (VOY: "Demon")
