Are there terms to distinguish between the different levels of Drupal Core? "Core" seems to cover over a multitude of things, encompassing all the modules that ship with the Drupal code base.  But there seem to be levels of "core"-ness, and I'm wondering if there is official terminology to distinguish between these. For example:


There is the code that is not part of any module.  Does "core" include this officially?  It's not clear to me.
There are required modules: without these (at least in Drupal 7) you can't really build a Drupal site.
There are all the default modules that are part of Core.  You can turn many of these off without a problem, but they're still considered "core".  
There are modules that are turned off by default (PHP Filter) and which may even be considered a bad practice to use under normal circumstances (PHP Filter, again).


If I were naming these levels, I might call them


The Drupal Kernel
Deep Core Modules
(Normal) Core Modules ("Core Modules" would include 2 and 4 as well)
Outer Core Modules


But it seems like Dries may have an official terminology already.  Is there such a thing?
I'm not aware of any official definition of the various levels.

I will however, referring to your suggestions, explain a bit about what levels there are.


The Drupal Kernel is actually a component in Drupal 8, which is a Symfony Kernel
"Deep Core Modules" could be the include files that are not even modules. Some files, for example database.inc, bootstrap.inc and module.inc are so fundamental that core cannot function without them. (There are some more .inc files however that reasonably could become modules, I don't know why they are not)
Core modules then, core parts that are written in the same style as contrib modules, but are still so fundamental that core wouldn't work without them. There's a system module that manages a lot of low level administrative functionality. The node module is (in D7) still required. Field sql storage cannot be turned of either.
Outer Core Modules, are all the other things that are shipped with core because they are commonly used, but not necessary for Drupal to function. The forum, taxonomy terms, the user interface for the field system etc.


Again, I'm not familiar with any official terminology, other than all of it being "Core". When people work on core, I only hear references to the individual "components" of core, as specific in the Drupal issue queue.

When I hear "actual core", I think of levels 1 - 3.