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.

Also known as

Some sources take straightness for granted, and refer to such as just a line segment.