A line is the locus of points such that the gradient (slope) between that point and one fixed point in the plane is a constant.
Technically, that definition does not include a vertical line because its gradient is not defined! You could get around that this by requiring that either the gradient is a constant or, if it is undefined, then the inverse gradient (dx/dy) is constant.
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a straight line ..
I believe that is the definition of a straight line.
Two parallel lines.
The perpendicular bisector of the line joining the two points.
A line in 2D and a plane in 3D A perpendicular bisector of the line connecting the 2 given points