Section 2.7.5 (page 83) of Operating System Concepts Essentials, 2nd Edition says (emphasis in original):
Windows is largely monolithic as well (again primarily for performance reasons), but it retains some behavior typical of microkernel systems, including providing support for separate subsystems (known as operating-system personalities) that run as user-mode processes.
Googling turns up a bunch of irrelevant online quizzes, but also this Wikipedia page, which mentions the term several times but doesn't appear to give a definition. Is this a standard term? What is a definition distinguishing it from other similar terms (e.g. "subsystems" from the above quote)?