A lot of sellers do seem to think it means Nasty Old Sh!te though!
Can be with or without packaging, just not mounted or used. The only exception I think to that would be a full bike that was built BITD, is still "new" and unused. As a bike it is NOS, but unbolt the parts and they are no longer NOS, because they have been mounted and unmounted. They'd be unused, but not NOS....
That's not the same as taking a lot of NOS parts, frame, etc. and building a bike today from all NOS parts. That bike would be new and unused, but not NOS any more....