shell n. [orig. {Multics} 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
   {incantation}s 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.