doc: clarify what a subsystem is

Clarify what a subsystem is, this has been missing the docs.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

doc/glossary.rst

@@ -140,6 +140,10 @@ Glossary of Terms
       A number of different :term:`SoCs <SoC>` that share similar characteristics and
       features, and that the vendor typically names and markets together.
 
+   subsystem
+       A subsystem refers to a logically distinct part of the operating system
+       that handles specific functionality or provides certain services.
+
    system power state
       System power states describe the power consumption of the system as a
       whole. System power states are represented by :c:enum:`pm_state`.

doc/introduction/index.rst

@@ -23,6 +23,14 @@ The Zephyr kernel supports multiple architectures, including:
 
 The full list of supported boards based on these architectures can be found :ref:`here <boards>`.
 
+In the context of the Zephyr OS, a :term:`subsystem` refers to a logically distinct
+part of the operating system that handles specific functionality or provides
+certain services. Subsystems can include components such as networking,
+file systems, device driver classes, power management, and communication protocols,
+among others. Each subsystem is designed to be modular and can be configured,
+customized, and extended to meet the requirements of different embedded
+applications.
+
 Licensing
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