Reference Guides » Undocumented Contexts
Undocumented Functions
Mathematica is a sprawling system and WRI loves to brag about the number of functions in it. Unfortunately it also often under-performs due to this sprawl. Targeted, highly-specialized functions get buried deep in the system, while the most general form of the function exists at the "System`" level.
Therefore, a lot of using Mathematica well comes from taking advantage of the nominally undocumented, unsupported parts of the system.
To make this easier, I've attempted to compile lists of useful undocumented functionality, separated by context.