the point at which one thing ends and another begins
Many products fall into the borderline between different product categories.
The forest's edge marked the borderline between the wilderness and the urban area, where the trees gave way to houses and streets.
The river served as the borderline separating the two neighboring countries, and a bridge was the only official crossing point.
In the desert, the shifting sand dunes made it challenging to determine the borderline between one dune and the next, as they seemed to blend seamlessly into each other.
