Noclip
The mode where the player's collision is switched off, so the camera glides freely through walls, floors and out into the void behind the level.
"Clipping" is game-programmer slang for stopping something from passing through a surface. Doom shipped in 1993 with the cheat IDSPISPOPD for walking through walls; Doom II renamed it to the easier-to-spell IDCLIP, and in Quake's console the command was simply noclip. The name stuck across the industry.
For developers it is a work tool: fly to the bug instead of playing your way there, check that the back of the set isn't visible. Players use it to find things never meant to be seen — unfinished rooms, single-sided textures, a sky that turns out to be a box. The internet folklore about "noclipping out of reality" into the Backrooms is the same idea, just with worse lighting.