Unsure of what you are asking!
But if the issue here is how to define a line segment, write what you do know and then reconsider "undefined terms."
A line segment is a straight line that connects a given starting point and given ending point.
If you consider a circle of radius 3 units, the radius can be thought of as the line segment connecting the center of the circle to any point on the circumference of the circle.
If the center of a given circle is at C(0,0) and a point on the circumference is given by R(3sqrt(2),3sqrt(2)), then AC is the line segment joining these two points. This line segment has length 3 and is in the first quadrant, with coordinates x=3sqrt(2) and y=3sqrt(2) describing the end point of the segment.