Definition:Line/Straight Line Segment

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A straight line segment is a line segment which is straight.

In the words of Euclid:

A straight line segment can be drawn joining any two points.

(The Elements: Postulates: Euclid's Second Postulate)

Thus a definition for straight line which is frequently encountered is:

A straight line is the shortest distance between two points.

This is all very well but it assumes that the line in question terminates at two particular endpoints.