Margin, extremity (4)
I believe the answer is:
edge
I believe this is a double definition.
'margin' is the first definition.
(synonyms)
'extremity' is the second definition.
(I know that edge is a type of demarcation)
(Other definitions for edge that I've seen before include "Move little by little to the border" , "Sharp ridge" , "Brim" , "Sharp side of a blade" , "Lip; advantage" .)