Without wishing to overcomplicate matters, one could simply define a straight line as:
"the shortest distance between two given points".
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I would claim that a straight line is slightly bend as we define straight from the horizon.
a straight line passing from side to side through the center
a line with two arrows on the end.
Linear functions can be represented by a straight line in space. One way to define a straight line uniquely is to use its slope (or direction vector) and any one point on the line.
It would not have sides because it would be a straight line. A straight angle is 180 degrees. Anything above that is a "Reflex"