As contrasted with kernel, the shell is the outermost programming layer of an operating system. In computers it's the part that generally interacts with user commands and feedback to the user.
“I can talk to my Sales Engineer about any piece of equipment, and he always steers me in the right direction. He truly knows what he's talking about, and he never tries to over-sell me on anything. If he worked in the Nashville area, I'd probably owe him about 20 dinners by now!” Michael Erin Woody, Brentwood, TN