ShyTorque,
Essentially, it is no more nor less than that stated in the Annex 2 text:
"...a landing to be made without undue hazard to persons or property on the surface"
The FAA have shied away from making a determination of a 'congested area' for a number of years for their own good reasons. The US courts have ruled on those occasions when it was considered that regulations were broached and prosecution had taken place.
ICAO on the other hand have attempted to put some flesh on the bones in their definitions of 'Congested hostile environment', and 'Non-hostile environment' - in sections a), d) and the note.
Congested hostile environment. A hostile environment within a congested area.
Non-hostile environment. An environment in which:
a) a safe forced landing can be accomplished because the surface and surrounding environment are
adequate;
b) the helicopter occupants can be adequately protected from the elements;
c) search and rescue response/capability is provided consistent with anticipated exposure; and
d) the assessed risk of endangering persons or property on the ground is acceptable.
Note.— Those parts of a congested area satisfying the above requirements are considered non-hostile.
Jim