Shell - Definition and Overview

Amongst other things, a shell is:

• An animal shell, the hard, rigid outer covering of an animal such as a mollusk or gastropod or tortoise or turtle. See also: shell (material).
• The hard outer layer of eggs.
• The hard outer layer of some plant fruiting bodies such as nuts.
• An operating system shell, such as a Unix shell command line interpreter in the Unix operating system.
• An electron shell (energy level), consisting of a number of electrons in an orbital around an atomic nucleus
• The short name for the Royal Dutch/Shell oil company.
• A projectile with explosive charge, shot from a gun; typically shells are larger than bullets and shot from larger guns. See shell (projectile).
• In physics, see On shell and off shell
• A structural form, usually made of concrete, that principally derives its strength by virtue of its shape. The term is derived from the primary usage in the context of nature. For example, the shell of an egg though very thin is difficult to break because of its shape.