From an architectural standpoint, we can break up Drupal into three pieces--its core, modules, and themes.
When we discuss Drupal 8 core, we can interpret it in two ways. A more restrictive interpretation sees it as a functionality covered in all the code it ships with, without modules and themes. The more widespread interpretation sees it as the total code base it ships with (out of the box).
Although the most widespread interpretation is the latter (not least because it differentiates all the functionalities its standard installation contains versus all others provided by contributed modules and themes), it is interesting to consider the first one as well, even if just for a minute. In this way, we can distinguish, ...
