In some video games, noclip is a PC game console cheat command that prevents the first-person player character camera from colliding with other objects and permits the camera to move in any direction, allowing it to go through such things as walls, props, and other players.
This can also happen in Half-Life engine games, where if a mapmaker has not set the correct properties on a floor (disable shadowing, etc), the shadows of the players above will show through. This is not helpful in the least for players in upper rooms trying to hide or avoid detection from players below them.
In Team Fortress 2 for The Orange Box, no clipping is used to build buildings in spawns, walk through walls, float everywhere, receive health, ammo and metal, and move players into enemy spawns.