What is Undefined?

In the last section we looked up the documentation 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 Attributes[Undefined]={Protected}.

Another place where you may be able to get information about Mathematica is StackExchange Mathematica and Wolfram Language

Next:

• Is there something similar to Undefined that we can use as an example?