At least two points are required to name a line segment.
line segment
straight line.
the diameter
a diameter
line segment, line ab, __ ab
The circle's diameter.
This is simply called a line. <---------------O-------------------------------O----------------------> But this is called a line segment: O-------------------------------------------O
You need two.
segment AB
Line segment
Line segment
For any line, there needs to be two points. If the line passes through points and extends in both directions forever, it's a line. If the line just goes between the two points, its called a line segment. If the line goes through both points but only extends in one direction, it's a ray.