An area of at least 30 or so feet (10 m) around a structure or maybe around you, cleared of flammable stuff, so the structure doesn't burn (maybe you too).
If you get into a situation while backpacking where you need defensible space, then you've made your campfire too big. Cut back already.
In case of free range fire (that is, one that is not your fault) then such a space may be one you make to save your hide, or a defensible space can be the result of a natural barrier, such as an ocean.
Very few fires get big enough to jump an ocean. On the first try.
Which should give you enough time to do some more running away.
Screaming is allowed in this sort of situation.