What is a line through a circle called?

A line that goes through a circle is a secant line. (Remember that a line is infinitely long.) Anyline that passes through a circle is a secant line, whether it passes through the center of the circle or not. Compare this to a line segment with endpoints on the circumference of a circle. That segment is called a cord of that circle. If the cord of a circle passes through the center of that circle, it is a diameter of that circle, which is the longest cord of the circle.