Clipping is a collision-related glitch that allows a fast-moving object to pass right through another object. The glitch is common in many games, including the Halo Trilogy, and its name is likely derived from the term, "noclip".
Clipping occurs when an object moves so rapidly that it passes through a wall, surface, or other object, without any collision occurring. This happens because of how collision detection works.
On every frame of animation, the game's collision detection engine checks all objects' positions. If two objects are touching or intersecting, then the game knows that a collision has occurred, and steps are taken to handle it.
The flaw, though, is that if an object moves extremely fast, then there may not be any frames of animation where it actually intersects a wall. On one frame of animation, it would be on one side of the wall; on the next frame of animation, it would be on the other side. Most collision detection engines do not check for movement between frames, so such an error would not be corrected, and the fast-moving object would continue its movement.
A typical frame rate for video games is 30 FPS. This means that extremely high speeds are required to clip through a wall. Halo 3 's Pan Cam, for example, can be used to clip through invisible walls in Theater, but only after it is accelerated to at least fifty thousand times the normal speed.