line segment
straight line.
the diameter
a diameter
The circle's diameter.
line segment, line ab, __ ab
This is simply called a line. <---------------O-------------------------------O----------------------> But this is called a line segment: O-------------------------------------------O
You need two.
segment AB
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.
Line segment
Line segment