Typically it is the body structure without the closures, ie doors etc. For some work it is important to specify whether it is glazed or not. If it is we call it the glazed BIW, but some people call the glazed BIW just a BIW.
Some companies call the BIW a body in primer, BIP.
Body in white refers to the structure prior to any priming and finishing operations. Generally applied to the body structure and not the sub structures (engine cradle, etc.).