(covers information from several alternate timelines)
The deuterium injector, also simply known as matter injector, was a component of the deuterium injection subsystem. Its counterpart was the antimatter injector, with the two together known as warp injectors as part of the matter-antimatter reaction assembly.
Deuterium injectors were located under the deuterium tank. The injectors precisely compressed and streamed matter (deuterium) through the deuterium stream coils into the dilithium regulation chamber. (TNG: "Contagion", "Booby Trap", okudagrams)
In 2366, Duffy and Costa were assigned to realign the magnetic capacitors of the matter and antimatter injectors aboard the USS Enterprise-D. Later, when the injectors had become locked due to invidium contamination, they were cooled down by gaseous cryonetrium to minus two hundred degrees celsius. The matter injector went online through the process, but the antimatter injector didn't until power was cycled through the ventral relay. (TNG: "Hollow Pursuits")
In 2374, while the USS Defiant was repaired under Jem'Hadar supervision, Kira Nerys claimed she was ready to connect the deuterium injector and ordered a Jem'Hadar to power up the plasma relay slowly. (DS9: "One Little Ship")
The Red Squad cadets often had trouble with the USS Valiant's deuterium injector start-up routine, as the system often experienced power spikes. When he came aboard in 2374, Nog suggested doing what Chief O'Brien had done on the Defiant, by recalibrating the lateral impulse control system. (DS9: "Valiant")
In 2377, Captain Kathryn Janeway proposed to trade zeolitic ore to the Annari in exchange for deuterium injectors. The deal fell through after the Annari found USS Voyager crewmembers had been helping their enemies, the Kraylor. (VOY: "Nightingale")