The built-in safeguards that stop holographic bullets, blades and falls from harming real people on a holodeck, and whose failure or deliberate removal drives many stories.
Holodeck safety protocols are the routines that keep a simulation from injuring its users. With them running, a holographic weapon cannot kill and the computer steps in before a fall or blow does real damage. Switching them off is restricted, and in Descent Data needs the agreement of two senior officers before he can do so.
The safeguards fail in The Big Goodbye, where a gangster's bullet badly wounds the historian Whalen, and they are overridden by events in stories such as A Fistful of Datas and Our Man Bashir.
They are also turned off on purpose. In Star Trek: First Contact Picard disables them so that a holographic machine gun in his Dixon Hill programme will kill two Borg drones, and in Extreme Risk B'Elanna Torres runs dangerous programmes without them as a symptom of her grief over the Maquis.