Borderline is an adjective and a noun.
As an adjective it can be used to show how something can be divided between two very different conditions, with the possibility of belonging to either one of them.
For example: A borderline case. A borderline candidate.
As a noun borderline shows something that separates two different qualities.
For example:-
The borderline between love and hate is very thin.
This photo shows the borderline between the former GDR and West Germany. Called the Innerdeutsche Grenze or Deutsch-Deutsche Grenze, it was an extensive system of fortifications that ran the entire 1,381-kilometer border.