The deuterium injector, or simply matter injector, was a component of starship's warp core which precisely compressed and streamed matter (deuterium) into the form which entered the dilithium articulation frame.
In 2372 in the mirror universe, Benjamin Sisko took the ISS Defiant's deuterium injector offline and online again. It required the flow regulators to be shut off. (DS9: "Shattered Mirror")
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, Nog suggested doing what Chief O'Brien had done on the USS 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 Voyager crewmembers had been helping their enemies, the Kraylor. (VOY: "Nightingale")
Later that year, The Doctor, functioning as the ECH, repaired Voyager's deuterium injectors among other systems that had been damaged by a subspace mine. (VOY: "Workforce")