shell [orig. Multics n. techspeak, widely propagated via Unix] 1.

shell [orig. Multics n.

techspeak, widely propagated
via Unix] 1. [techspeak] The command interpreter used to pass
commands to an operating system; so called because it is the part
of the operating system that interfaces with the outside world.
2. More generally, any interface program that mediates access to a
special resource or server for convenience, efficiency, or
security reasons; for this meaning, the usage is usually `a shell
around' whatever. This sort of program is also called a
`wrapper'. 3. A skeleton program, created by hand or by another
program (like, say, a parser generator), which provides the
necessary incantations to set up some task and the control
flow to drive it (the term driver is sometimes used
synonymously). The user is meant to fill in whatever code is
needed to get real work done. This usage is common in the AI and
Microsoft Windows worlds, and confuses Unix hackers.

Historical note: Apparently, the original Multics shell (sense 1)
was so called because it was a shell (sense 3); it ran user
programs not by starting up separate processes, but by dynamically
linking the programs into its own code, calling them as
subroutines, and then dynamically de-linking them on return. The
VMS command interpreter still does something very like
this.