From Halopedia, the Halo wiki
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.
One common and well-known example of the common definition is on the Multiplayer map Danger Canyon in 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 the 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 Karo'etba-pattern 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. This is unofficially referred to as 'Ghost Hacking' within the gaming community when the glitch is performed with a Ghost.
A single player can also glitch through the wall by strafing sideways toward it with a Ghost. Right before a collision occurs, the player should dismount the Ghost; the vehicle will force them through the target surface. Also in Halo: Combat Evolved sometimes a banshee can hit a player so hard that it will force them through the ground where they will fall for several minutes below the game, This is most common on the multiplayer map Blood Gulch.
In Halo: Combat Evolved, within the ledging category 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; on Blood Gulch, on two ledges behind the Blue base.
In 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 rag doll animations, used when a player is killed.