It is called a border.
The portion of a line that falls between two points is called a line segment.
A line segment
the measure of a straight-line distance between two points is called length.
the distence between two point called line.
It is called a border.
The Border between the two countries separating Russia and China is just that a Border line.
border ..
borders
A line between two verticies is called a line segment.
The portion of a line that falls between two points is called a line segment.
The line is called the terminator
The name of the line on a map that separates two countries is typically called a border or boundary.
India and china
It's the line (boundary) between two countries.
It is called a vinculum.
-transform boundary -convergent boundary -subduction zoone -divergent boundary -mid-ocean ridge -rift valley ...these are some possible answers to your question.