In the last section we looked up the documention for
Undefined. What exactly is it?
To find it in the WMA docs we add the name after
https://reference.wolfram.com/language/ref/ which gives https://reference.wolfram.com/language/ref/Undefined
It is listed as a “Built-in Symbol”. In contrast to the other examples and indeed most of the kinds of things you are likely to add, this one is not a function, but a “symbol”. The description for this is “a symbol that represents a quantity with no defined value”.
Asking in the Q/A section of mathics-core we learn that
Another place where you may be able to get information about Mathematica is StackExchange Mathematica and Wolfram Language