Deuterium, (chemical symbol 2H) also known as heavy hydrogen, is a stable isotope of hydrogen. Deuterium has a single proton and neutron. It occurs in less than one-half of one percent of naturally-occurring hydrogen.
Deuterium is used as a fuel in warp reactors, it is injected into warp cores along with its antimatter counterpart, anti-deuterium. The annihilation of the two substances within the core releases the energy used to power warp drives. For storage purposes, deuterium is frozen into a pellet form, and stored in tanks on board starships. In Federation Galaxy-class starships, deuterium is stored in a large tank at the top of the secondary hull.
In the Federation, deuterium is supplied to starships and other installations via fuel carriers. Starships would also supplement their supplies of deuterium by the use of Bussard collectors, which gather deuterium and other interstellar particles for use on board ships. (TNG reference: Star Trek: The Next Generation Technical Manual)
While performing a survey of a planet in the Hobus star system in 2387, Ayel, science officer of the Narada, detected trace amounts of iridium and deuterium, as well as large deposits of decalithium. (ST - Countdown comic: "Number One")