Along the same lines, cheating also includes replacing player models or any other in-game entities with substitutes, or editing maps to place holes/windows in otherwise solid walls. A common hack in Doom II that has since been antiquated was replacing the default player models with a huge-ass models (such as the Cyberdemon in Doom II), whose body parts are so large that they show right through the other side of the walls!
I've seen similar cheats before too. During PGL Quake Teams, the officials used hacked models (which had sticks coming out of their bodies) to observe games, as it's easier to determine whether or not a player you're watching is using a similar "pak2." In Quake II, it was possible to delete all of the player models from the baseq2 directory as well, making anyone using those models appear as a fullbright white rhombus, which is much more visible than standard. Also, since the various pain sounds are proportionally mapped to the health of the player, it was very possible to go in and replace the sounds with more easily discernable ones. (i.e. changing the 30 health "ugnhh" sound to say "player has 30 health."
The Grey Area
That's all fine and dandy, though the issues we've covered so far are generally more black and white. Now let's cover some of the more controversial stuff, such as console commands. While most client-side modifications are pretty harmless, there's a lot of potential here for some serious abuse as well.
Well, my policy has always been that if it's allowed as a client-command in the console, it's OK. Those folks adventurous enough to work their way through the console finding tidbits such as v_kicktime, cl_bob, r_rollangle, gl_modulate, and FOV should be allowed to take advantage of them. These commands don't change any gameplay aspects, and don't allow you to see or know any information you're not otherwise privy to.