What defines a shell?

What exactly defines a “shell”? Does it mean it does not have a bathroom and kitchen? I’m looking at several properties that are in need of serious rehab and pretty much need to be totally gutted but are not listed as shells. Also, some properties listed as shells seem to be cheaper than ones that supposedly only need “cosmetics”. I’m confused! Thanks for your help!

Here is a good definition:

A “shell” is a rowhouse that has the brickwork and roof and structure still in good shape, but the interior could range from completely gutted to very tatty to very decent.

The term “Shell” does vary a little from area to area. However in most places it is simply a structure with nothing but four walls, foundation, roof and maybe some interior walls. It almost always needs to have a complete gut rehab.