The word "discriminated" means that at run time, if you receive a value whose type is a union of types you can discriminate which is the actual type of the value, so you can use that value in expressions that expect a specific type, not a union of types.

This is in contrast with what is called "union" in the C language, where you must know a priori the type of a value in order to use it correctly.

Moreover, if you can discriminate at run time which is the actual type, that means that you can discriminate between values that happen to have the same representation, but which come from distinct types, e.g. if you had a union between "signed integer" and "unsigned integer", you could discriminate between a signed "3" and an unsigned "3".

This property ensures that the number of possible values for a discriminated union type is the sum of the numbers of possible values for the component types, hence the alternative name "sum type", unlike for a non-discriminated union, where the number of possible values is smaller, because from the sum you must subtract the number of possible values of the intersection sets.

The C# unions described in the article are discriminated unions, except that their component types are not the types literally listed in their definition. If some of the component types are optional, than the true union components are the corresponding non-optional types, together with the null a.k.a. void type, which I find as a rather strange choice.