Wall hacking

Wall Hacking is the use of physics glitches in Halo: Combat Evolved and Halo 2 to force objects through walls and barriers. It can also refer to a subdivision of ledging in Halo: Combat Evolved that involves ledging in a wall that is sloped inward, or where the player is no longer standing on a ledge, but floating between ledges.

Common Definition
One common and well-known example the common definition is on the Multiplayer map Danger Canyon on Halo: Combat Evolved. Because this map is arranged in a circle, with the two bases next to each other (albeit with a wall in between), if a player passed through the wall, they would end up inside opposing team's base. People have done this in Team CTF games when teamkilling was disabled by "ramming" another teammate through the wall with a high-speed land vehicle, like a Ghost or a Warthog. The "rammed" player gains enough speed to fly through the wall and pop out at the other base. Sometimes the player does not have enough velocity to travel through both walls, in which case they will fall through the map and die.

Ledging Definition
In Halo: Combat Evolved, within the ledging catagory of stunting, there is a subdivision known as Wall Hacking. Wall Hacking, contrary to the well known cheating term, involves ledging within a wall where the majority of Master Chief's body is not in the wall. (The feet or legs are in the wall, but the upper body is sticking out.) The second definition involves the Chief hanging by his upper body or by his head from a ledge. Common examples of Wall Hacking can be found on Damnation, within the Hallway Ledge when you fall between the Ledge and the wall; and on Blood Gulch, on two ledges behind the Blue base.

Halo 2
In Halo 2, certain maps have spots where, if players are thrown with enough force at a certain angle or speed, they will fall through the map. This usually only happens with ragdoll animations, used when a player is killed.