The Wikipedia entry reads
In computing, a shell is a user interface for access to an operating
system's services. In general, operating system shells use either
a command-line interface (CLI) or graphical user interface (GUI),
depending on a computer's role and particular operation. It
is named a shell because it is the outermost layer around the
operating system.[1] [2]

Rich’s sh (POSIX shell) tricks
Discussion on Lobsters