Definition - What does Undocumented Feature mean?
An undocumented feature is a function or feature found in a software or an application but is not mentioned in the official documentation such as manuals and tutorials. Sometimes this is due to pure oversight, but sometimes the feature is undocumented on purpose since it may be intended for advanced users such as administrators or even developers of the software and not meant to be used by end users, who sometimes stumble upon it anyway.
Techopedia explains Undocumented Feature
Undocumented features are often real parts of an application, but sometimes they could be unintended side effects or even bugs that do not manifest in a single way. Sometimes they are meant as Easter eggs, a nod to people or other things, or sometimes they have an actual purpose not meant for the end user. Some undocumented features are meant as back doors or service entrances that can help service personnel diagnose or test the application or even a hardware.
Undocumented features can also be meant as compatibility features but have not really been implemented fully or needed as of yet. Users often find these types of undocumented features in games such as areas, items and abilities hidden on purpose for future updates but may already be functioning in some extent. Microsoft developers are known for adding Easter eggs and hidden games in MS Office software such as Excel and Word, the most famous of which are those found in Word 97 (pinball game) and Excel 97 (flight simulator).