From https://golang.org/ref/spec#Types:

Named instances of the boolean, numeric, and string types are predeclared

I read this as "defined types are predeclared for the boolean, numeric and string types and are identified by int, int32, bool, string etc"

Hence they are defined types.

But the sentence that follows in the spec confuses things:

Other named types are introduced with type declarations

It is, I think, a hangover from pre-alias days, as was picked up in this comment

@griesemer @mdempsky - should that reference to "named type" now be "defined type"?

Yes, int, etc are defined types.

As @myitcv points out, we used to just say "named" type. Now that there are type aliases (which have names but may refer to an underlying anonymous type literal), we instead say "defined" type to refer to named types that aren't aliases.

We still have the notion of a "named type": Like before (the split into defined and alias types) it's a type that has a name. Unlike before, having a name alone doesn't give it special meaning; we also have to look at its declaration. We need the notion of a named type in other places, such as where a type name is needed.

The predeclared types are named types (they have a name), and most of them (expect for byte and rune) are defined types. It's true that they are somewhat special insofar as their underlying types are themselves.

We could call the predefined types but that would require yet another term to be introduced. Instead, I opted for simply stating explicitly which predeclared types are defined types and which ones are aliases. CL forthcoming.

Change https://golang.org/cl/64591 mentions this issue: spec: state which predeclated types are defined or alias types (clarification)