shell

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'.