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 (whereas the most common isotope of hydrogen lacks any neutron particle). Naturally-occurring hydrogen consists of less than 1% deuterium. The antimatter counterpart of deuterium is antideuterium.
Deuterium with an admixture of tritium was used as fuel in matter-antimatter reactions aboard starships, like the USS Enterprise-D. It was stored in the deuterium tanks. Federation starbases had reservoirs of deuterium which were used for refilling the deuterium tanks of starships. (TNG: "Contagion", "Booby Trap", "Disaster" okudagrams; TNG: "Captain's Holiday")
Aboard the Galaxy-class USS Enterprise-D, in the Engineering Systems Database, there were folders titled 'Deuterium/Tritium Loading Procedures' and 'Deuterium/Tritium Tankage Transfer and Safety'. These folders contained technical manuals relevant to the topics. (TNG: "Booby Trap" okudagram)
The Bussard collectors on some starships were used to collect hydrogen and other 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", "Bliss")
In 2369, Chief O'Brien ordered that the deuterium flow to carbon reaction chamber two should be increased by five percent, however the station's computer refused to do this and told O'Brien to read the Cardassian operational guidelines. (DS9: "The Forsaken")
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. (DS9: "Civil Defense")
Deuterium was scarce within The Void, and a number of vessels within it raided others in order to obtain it. Garon had a saying that "...good intentions are like deuterium reserves; they tend to get lost in The Void." Despite this, he joined The Alliance in helping protect ships against such attacks. (VOY: "The Void")