Developer console
A command line built into the game that drops down over the screen and lets anyone who knows what they're doing change almost anything while it runs.
Quake made it standard in 1996: press the key below Esc and a translucent text terminal slides down from the top of the screen. From there you could load maps, bind keys and tweak console variables, cvars, such as gravity or field of view. The whole configuration was saved as a text file of those same commands, which made the game scriptable long before anyone talked about modding APIs.
For developers it is the cheapest debugging tool going — a new tweak costs one line of registration instead of a menu. Source, Unreal and Bethesda's engines carried the idea forward. In shipped games it is usually hidden behind a launch flag, since players would otherwise find noclip within the hour.