demarcation

US /ˌdiː.mɑːrˈkeɪ.ʃən/
UK /ˌdiː.mɑːrˈkeɪ.ʃən/
1.

sự phân giới

a border or a rule that shows the limits of something or how things are divided:

:
The river serves as the line of demarcation (= the line showing the separation) between the two counties.