The radius
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That's called a chord.
A straight line joining points on a circle is called a "chord" of that circle. If the line happens to pass through the center of the circle, then it's a "diameter" of that circle. The question asked about "points" on a circle, so two points on the circumference of that circle are being considered. (No line can join more than two points of a circle.)
Isohyet
Median line.
This is called the diameter of the line that passes through the center of the circle. Half of that line is called the radius.