1. A text-mode window containing a command line interface to the operating system. From I-gloss

2. One of several command line interfaces available on Unix machines, some common shells include Bourne shell, ksh, and tcsh. From KADOWKEV

3. The default command-line interface on UNIX systems. Key point: This is similar to the "Command Prompt" or incorrectly named "DOS Prompt" on Windows systems. Key point: Many systems pass filenames along with commands directly to the shell. Hackers can exploit this by sending special shell characters (like the pipe | character) as part of filenames in order to execute their own commands. This is an example of an input validation exploit. Examples of this are web-servers, PERL scripts, and CGI scripts. Key point: The most popular shell among hackers is probably "bash", the shell from GNU that ships with Linux. (Culture: The original shell on UNIX is known as the "Bourne Shell", named for its creator. The acronym "bash" means "Bourne Again SHell", reflecting that fact that it is a rewrite of that shell). Key point: Retrieving someone's .bash_history file is a common attack against UNIX machines. Several embedded systems have shipped such that the file http://raq.robertgraham.com/~root/.bash_history could be retrieved via the web. Key point: The holy grail of UNIX hacking is to somehow obtain (or re-obtain) a root shell. In other words, the hacker wants to get a command-line on the victim system in order to carry out any task. For this reason, buffer overflow exploits often contain what is called "shell code". When the victim process is running with root privileges, the buffer-overflow will cause that process to begin running a shell. For example, an exploit might send a long password containing the shell code to an FTP server, converting the TCP connection to the FTP server into a full command-prompt from which any program can be launched. From Hacking-Lexicon

4. [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. From Jargon Dictionary

5. a utility program that enables the user to interact with the UNIX operating system. Commands entered by the user are passed by the shell to the operating system which carries them out. The results are then passed back by the shell and displayed on the user's display. From Linux Guide @FirstLinux