A BAD is a place where one or more people can (over)live a disaster (natural, plague, war, economic collapse ... put here the reason that pleases you or scares you the most), for a more or less long period of time, without needing the outside: supermarkets, hospitals, and other services present in a modern society.
This definition is a little broad, in fact a BAD will often materialize by a place in the countryside where one will try to be as autonomous as possible on the most important aspects of survival:
