In regard to the Doom engine, limit removing refers to increasing or removing static limits inherent in the original executables and the Doom source code, which can be considered bugs by players or WAD designers.
WAD designers sometimes describe their levels as requiring a limit removing engine. Essentially, this means that the level uses no features which are not present in the vanilla Doom engine. However, the level must usually still be run with a source port, typically because the level is too detailed or large to run using the standard executables, as with it the game will eventually or immediately terminate or crash, or suffer from visual glitches or other anomalies. Because the level does not use any extra features not available in the unmodified engine, players are free to use any of the many engines that raise Doom's inherent limits.
The term "limit removing" is only partially accurate, as limits are not always completely removed due to format requirements and other considerations, and different engines raise the limits to different degrees. In many cases "limit removing" corresponds with or implies the limit extensions of the Boom engine (not including Boom-specific mapping features), although newer source ports have raised various limits even further, and even the original executables can be hacked to allow higher limits (see Doom32 and UDoom32).
For a list of limit-removing source ports, see Category:Limit removing source ports.