clipping can also mean when two solid objects within the game overlap one another, like you see a NPC head fall into the side of a building which shouldnt happen in real life. When you type no clipping as a cheat it allows you the freedom to pass through all objects regardless of their solid/mass state within the 3D world (which includes the "gravity" that is keeping you from flying around when this cheat is not enabled). Turning the no_clipping on and off is useful when designing custom maps as it allows for a "birds eye" view of the world you have created.

Hope this helped!

And welcome to hell butt-munch!

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