Invisible wall
From Halopedia, the Halo wiki
Invisible Walls are a term used by many Halo 2 players who do tricks and glitches, usually involving exiting to the outside of a level. In most cases it is impracticle to try to get through one in an actual game when your trying to win. The invisible walls act like any other wall, with the exception that they are of course invisible and usuall encircle the map. Often times just outside of an invisible wall is a kill barrier, which is best described as a zone in which once you walk into it, you die suddenly for no appearent reason, although most of the time it says you fell to your death. There are ways up and or through these invisible walls, however as climbing outside of the level in a match is considered cheating, no details will be given at this time.