The river serves as the line of demarcation (= the line showing the separation) between the two counties.那条河是两国的分界线。
In some schools there is little demarcation between subjects (= subjects are not taught separately).在某些学校里,各学科之间几乎没有明确的界限。
On this map, demarcations between regions are shown with dotted lines.这张地图用虚线标出了地区分界线。