The term Shell is associated with the Operating System's interface that allows to access to the services of the system itself. Shells can be graphical (GUI - Graphical User Interface) or command line (CLI - Command Line Interface).

The Shell is the most external and visible part of the OS, the part which is directly available to the user.
Usually the term Shell defines the system interaction textual tool (CLI), which is often used in the Linux world and, in general, on server OSes.