Border, boundary (4)
I believe the answer is:
edge
I believe this is a double definition.
'border' is the first definition.
(synonyms)
'boundary' is the second definition.
(edge is a kind of boundary)
(Other definitions for edge that I've seen before include "Guitarist with U2" , "Margin, fringe" , "Margin, outside limit" , "Rim or sharp side of knife" , "Steal" .)