The heavy isotope of hydrogen that serves as the basic fuel of Federation starships, feeding fusion reactors and supplying the matter side of the warp core's matter-antimatter reaction.
Deuterium is a real isotope of hydrogen and, in Star Trek, the everyday fuel of starships. Stored as a super-cold slush in large tanks, it powers impulse fusion reactors and is the matter that meets antimatter in the warp core.
It is referred to throughout The Next Generation and later series, but it matters most in Voyager, where the ship is far from any starbase. In the episode Demon a severe shortage forces Voyager to land on a hostile Class Y planet in search of deuterium, where the crew meet the mimetic silver fluid that copies them.
Voyager also encounters deuterium as a trade good in the Delta Quadrant, and the crew ration power and replicator use to conserve it.