Mode of many FPS games which allows the player to walk through walls. Generally no clipping also makes the player unable to pick up any items as collision detection is turned off. This is loads of fun if your game is hacked to allow codes in multiplayer mode. In DOOM you enter this mode by going IDSPISPOPD, DOOM 2 is IDCLIP and I'm pretty sure it can be done in Quake and many other FPS games.

Don't go past the edge of the map! Ahhhhhh, video feedback!