A regeneration inhibitor was a type of weapon which could prevent Time Lords from regenerating. Stasers were one such weapon used by the Chancellery Guard on Gallifrey, with a single shot on the most lethal setting being capable of both killing a Time Lord outright and disrupting the subsequent regenerative process.
Daleks developed regeneration inhibitors during the Last Great Time War, incorporating them into their gunsticks. (AUDIO: The Conscript) Varga plants were also engineered as regeneration inhibitors capable of killing Time Lords with a touch. (AUDIO: Legion of the Lost)
Regeneration inhibitor weapons were possessed by the Kovarian Chapter, whom intended to kill the Doctor. With a firearm capable of producing double blasts to take out a binary vascular system, Brooke killed H-One as well as Andrew Edwardson, the latter of whom served as a decoy for the Fifth Doctor. (AUDIO: My Dinner with Andrew, The Furies)