Wall Hacking is the use of glitches in the Halo and Halo 2 physics engines that allows players and objects to go through walls and barriers. It can also refer to a subdivision of Ledging in Halo 1 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 you flew through the wall, you would get to the opposing team's base. People have done this in Team CTF games when Team Kill / Friendly Fire is turned off in the game settings by 'ramming' another teammate through the wall with a high-speed land vehicle like a Ghost or a Warthog. The 'rammed' player has 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, they will fall through the map and die.
Ledging Definition
In Halo 1, 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 defintion involves masterchief 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 certain spots where, if you are thrown with enough force at a certain angle or speed, you will fall through the map. This usually only happens with ragdoll animations, used when a player is killed.